This project investigated the effects of process capability, line station operation efficiency, and line balance on the product throughput of a wooden door manufacturing process using time-and-motion study and simulation methods. The problem-solving approach DMAIC was used to identify root causes to the problems and implemented the process improvements for the production line efficiency and throughput based on lean manufacturing concepts. The investigation results indicated that the main operation of the wooden door manufacturing process was based on manual and semi-automatic operations. The process capability of whole operation before implementing process improvements was around 600 doors monthly. The coefficient of variation of process time for manual operation ranged from 12% to 23%. The semi-automatic operation ranged from 2% to 20%. The average station efficiency and throughput was 31% and 20 doors per day respectively. After implementing the removal of obvious motion waste, the processing time of the whole process reduced 22.9%. The reduced process time variation for manual operation ranged from 9% to 14%, and the semi-automatic operations ranged from 3% to 7%. The final product output had a 200% increase and the total process efficiency had 100% improvement after implementing the improved process flow design. Meanwhile, the average station efficiency and output increased to 61% and 65 doors per day, respectively.