The aim of this research is to reveal more information and understanding on behaviour and failure mechanisms of high strength steel endplate connections (combining high strength steel endplates with either mild steel or high strength steel beams and columns in endplate connections) in fire and after fire, for an effective application of high strength structural steels in civil engineering as well as enhancing the fire safety of steel structures. The research work consists of three main parts: (Part I) numerical validation of the research idea; (Part II) experimental study on mechanical properties of high strength structural steels in fire and after fire; (Part III) full-scale behaviour of high strength steel endplate connections in fire and after fire. In Part I, the research idea of combining high strength steel endplate with mild steel beam and column in endplate connections is proposed and validated. A numerical modelling of endplate connections using ABAQUS/Standard was conducted, to reveal how high strength steel endplate connections behave in fire. The proposed FE modelling was further validated with fire test results on mild steel endplate connections reported by the University of Sheffield. On this basis, the performances of HSS endplate connections at ambient temperature and at elevated temperatures were further predicted and compared with those made of mild steel. It is found that the proposed research idea, combining HSS with mild steel in connections, may be used for further investigations of improving the behaviour of steel connections under fire conditions as well as promoting the application of HSS in civil engineering. Hence, the subsequent research is carried out . In Part II, the experimental study on mechanical properties of high strength structural steels in fire and after fire is presented. Firstly, in order to provide convincing proof for safe fire-resistance design of high strength steel structures and validate the limited available research results in literature, an experimental research was performed on S460, S690 and S960, using both the steady state test method and the transient state test method. The elastic modulus, yield strength and ultimate strength of these three high strength structural steels at elevated temperatures up to 700°C under various conditions were obtained and compared with available literature and the recommendations of current leading design standards for steel structures (i.e. EC3, AISC, AS 4100, ASCE and BS5950). The comparison of HSS with mild steels shows that the deterioration of mechanical properties of structural steels at elevated temperatures is dependent on steel grades and manufacturing method. Comparison with current European, American, Australian and former British design standards for steel structures shows that no current design standard may be used conservatively to conduct fire-resistance design of steel structures with high strength steel S460N, S690 or S960. Therefore, some unique predictive equations calculating reduction factors of mechanical properties for HSS S460, S690 and S960 at elevated temperatures were proposed and recommended for safe practical design and structural analysis. Moreover, a series of experimental studies were undertaken on three high strength steel grades S460, S690 and S960, to investigate their post-fire mechanical properties after cooling down from fire. Steady state tensile tests were undertaken after cooling down from various different fire temperatures up to 1000°C, to reveal the residual elastic modulus, yield strength, ultimate strength and post-fire stress–strain curves of S460, S690 and S960 after fire. The results showed that the steel grade has a significant influence on the post-fire residual mechanical properties of structural steels. Further, some separate predictive equations were proposed to determine the post-fire elastic modulus, yield strengths and ultimate strengths of S460, S690 and S960 respectively. Finally, the behaviour of HSS endplate connections in fire and after fire is studied via experimental tests and numerical analysis in Part III. The full-scale tests on high strength steel endplate connections in fire and after cooling down from fire were carried out to reveal their characteristics. Their performance were compared with mild steel endplate connections. It is found that a proper thinner HSS endplate can enhance the connection’s rotation capacity at ambient temperature, in fire and after fire (which guarantees the safety of an entire structure), and simultaneously achieve almost the same moment resistance with a mild steel endplate connection. In addition, a numerical study on HSS endplate connections in fire and after fire was conducted using ABAQUS/Standard. Validation of the numerical modelling against all representative experimental results conducted on moment-rotation relationship, failure mode and yield line pattern of connections showed good agreements. Hence the numerical analysis method can be used with confidence to predict the behaviour of high strength steel endplate connections under various fire conditions as well as at ambient temperature, and after cooling down from fire. All in all, the mechanical properties of HSS S460, S690 and S960 in fire and after fire as well as the experimental and numerical study on high strength steel endplate connections in fire and after fire have been presented in this thesis. It offers a basis for structural engineers to conduct fire-resistance design of steel structures with high strength steels, and to perform accurate evaluation and safe reuse of constructional structures with members made of S460, S690 and S960 after fire as well.