Optimal design of support structure including foundation and turbine tower is among the most critical challenges for offshore wind turbine. With development of offshore wind industry heading for deeper ocean areas, new support structural concept such as a full lattice tower could be proven to be more advantageous than others when consideration of cost, safety and even environment aspects, etc are taken into. This thesis first gave introduction of present industrial applications of hybrid support structural concept combing a lattice foundation and monotower in relatively deep water areas before the presenting and introduction of challenges with the transition piece component. Conceptual model of transition piece design for a full lattice tower support structure proposal was discussed extensively which included consideration of structural form, functional requirement, mechanical condition, etc. A mechanical model of transition piece with regards to boundary condition and load conditions was also provided. Design and analysis of two different types of transition piece models under various load conditions were performed during preliminary design and with conclusion drawn, a refined final design of transition piece model for the full lattice tower support structural concept which has also included more practical aspects was assessed through investigation of its performance under varying load conditions and different load cases. This refined final design was found to be the most optimal design fulfilling all relevant requirements at a comparable structural cost. Conclusion and recommendation was therefore given in the last part. This thesis work is serving for a novel proposal of support structural concept for future offshore wind industry and relevant present experience is in fact nonexistent. The work applied present industrial information of hybrid support structure as basis along with consideration of offshore wind turbine structural and operational mechanism and their respective requirement. Structural analysis of transition piece design was conducted by means of finite element analysis technique and structural load conditions were simulated through up-to-date numerical modeling code for offshore wind turbine structure. Due to absence of relevant verification sources, advice and correction of the thesis content is much appreciated.