Since the six sigma strategy was first introduced to Motorola in 1987, it has been taken as an important business strategy to strengthen the competitiveness of leading companies in the global competitiveness environment. This thesis presents a six sigma project to reduce the cycle time of assembly line in a medium size automotive part company. The project follows a disciplined process of five macro phases: Define, Measure, Analyze, Improve, and Control. A CTQ is determined based on COPQ analysis, and a process map is utilized for identifying process input variables. Three KPIVs(key process input variables) are selected; tensile strength, width of welding bid, depth of welding bid, and DOE (design of experiments) is utilized for finding the optimal process conditions for three KPIVs. As a result of the project, the cycle time becomes 30 sec./unit from 56.3 sec./unit at the beginning of the project.