The purpose of this research is to reduce the defect rate from manufacturing process of in-mould decoration (IMD), a process combining PET film with decorative patterns are moulded together with ABS resins, which has been developed to reduce the secondary process of decorative screen printing. Recently, the case study company has produced components with a high defect rate average 11.43% in Feb-Jun 2018, which increased sharply to 22.3% in June 2018. The major causes of the defect resulting from PET film peeled off and difficult to assembly. In order to reduce the defect rate, the five steps of the DMAIC Six Sigma methodology are implemented. The detail of the IMD manufacturing process was studied to identify appropriate measurement methods and factors affecting PET film peeled off by using the cause and effect diagram and its scoring matrix. Then the prioritized factors were analyzed and selected from FMEA to conduct the design of experiment (DOE) to find significant factors and optimal parameter settings for improvement and control process to prevent reoccurrence. After improvement of PET film peeled off by setting appropriate parameter as well as implementing control plans and Standard Operation Procedure (SOP) to eliminate other defects from in-mould decoration process, the result shows that the defect rate of in-mould decoration process decreased from11.43% average in Feb-June 2018 to1.25% after improvement in July-Oct 2018. It is found that IMD peeled off defect is reduce 86.6% and warpage defect is reduce 97.8% of defect before improvement.