Flange-prop shaft coupling is used to connect drive shaft and transmission shaft. The original process was to use center hole as machining datum and then drill three holes first. However, the center hole and the three holes didn’t true located in the position it should be. It resulted in the deviation of true position. In order to meet the specification, the process has changed. This change made a big difference in comparison to the previous process. It not only improved the deviation of true position but also a breakthrough to the machining skill. The project ends with a success.