This research aims to demonstrate Six Sigma methodology to reduce defective rate. A case of glass-mold defective reduction in plastic lenses manufacturing was studied. After applying systematic DMAIC steps and statistical tools suggested by Six Sigma approach, it was observed that the defective rate due to scratched molds was decreased from 2,512 ppm to 826 ppm, and the sigma level was improved from 4.31 to 4.65. In addition, the improvement could save the production cost up to 75,000 USD annually.