This research mainly investigates the relationship between the process condition, the microstructural evolution and the mechanical properties of the low carbon S690 steel in complex WAAM deposited structures. Besides, the optimum process parameters for improving the quality of the multi-pass multi-layer structure are also the research topic in the thesis. With a decreased heat input, achieved by a higher travel speed or a lower wire feed rate, the cooling rate became higher in the single bead. A larger amount of martensite formed in the bead due to the higher cooling rate, which led to a higher hardness. When the heat input became smaller, less metal was deposited per unit length, which decreased both the width and the height of the bead. Two-layers structures were deposited with the different process parameters, and their qualities were evaluated based on defects, surface roughness, and geometry. Step over increments were set to two-thirds of the width of each bead to improve the surface quality. Wire feed rate 4.5 m/min and travel speed 8 mm/s are the optimized parameters to fabricate the sample with the highest quality. Functional grading was achieved in a component by depositing with different process conditions. In the functionally graded component, the high heat input zone, having a lower hardness, included a large fraction of ferrite and bainite because it was fabricated with higher heat input, resulting in the lower cooling rate. The low heat input zone, having a higher hardness, included a large fraction of martensite due to higher cooling rate, caused by the lower heat input. The microstructure in the low heat input zone was not homogeneous due to the occurrence of the softer white line region, including a large amount of ferrite. The crossing structures fabricated with the interlayer temperature of 100 °C and the interlayer time of 90 sec showed the similar microstructures and hardness result. Due to the impact of a cold substrate and thermal accumulation, the varying microstructure existed from the bottom region to the top region, leading to the hardness level along the building direction became: Bottom region > Middle region ≈ Top region. The higher cooling rate at the centre was observed because there was more area for heat produced at the centre to diffuse.