Quality control is the main factor in maintaining and increasing the level of customer satisfaction. PT K is a manufacturing company produced commercial vehicles in Indonesia. One type of vehicle produced is Colt Diesel Truck. Welding processes assembly of a vehicle cabin with portable spot gun and brazing welding machine that controlled by the operator. Based on defects historical data on Welding process from July to September 2020 production of Colt Diesel vehicles the proportion of defects was 4.13%. Using Six Sigma method with DMAIC methodology (Define, Measure, Analyze, Improve, and Control) to improve the quality. The tools used are SIPOC diagram, CTQ (Critical to Quality) in the Define stage; Control chart, DPO, DPMO, and sigma level in the Measure stage; Pareto Diagram, Ishikawa Diagram, Five Whys Analysis method, FMEA (Failure Mode and Effects Analysis), and FTA (Fault Tree Analysis) in the Analysis stage. There are five defects: dents, holes, minus spots, ding, and stray spots, with sigma level of 3.89752. Results of the Pareto diagram, the dent has the highest defect 86.2%. Based on the FMEA and FTA methods, three causes with the highest RPN value and probability are the machine handling process by operators who are not careful in the main body 1 welding process causes dents on the back panel and outer pillar, bolt door panel installation and machine handling by operators who are less careful in the door installment process which causes dents in the door panel. In improve stage, the improvement that can be given is to control the performance of the operator using a more effective feedback check sheet and to use poka yoke torque wrench in the door installment process. The application of the proposed check sheet feedback for 7 days production increases the sigma level to 3,9275.