In recent years, it has to spend lots of time and effort to get the certification of CMMI. Therefore, everyone is looking to tools or methods for speeding up the CMMI certification. CMMI level five, causal analysis and resolution, is an important issues for all industries. In the process of software development, we have to identify the causes for defects at first. Then, it uses a systematic approach to sum up the necessary causes for software defects in order to help managers make better decisions and develop action items. With no doubt, it is a very important issue in the process of software development. This study aims to explore the subject of using the methods of causal analysis and resolution to solve the problems of software defects. Through the implementation of CAR, we can determine the root causes of defects and avoid importing defects to products. This study focus on the implementations of CAR and it proposes the methods, procedures and management forms. Moreover, this study will introduce the Mill¡¦s methods for causal reasoning used in the structure of CAR. Therefore it can help managers with a better way to sum up the causes for defects. The study uses case study method. Firstly, it connects the company for data collection of cause and effect diagram and combines the Mill¡¦s methods to inductive causal and analysis. Then it arranges interviews with the company managers to identify the necessary causes of defects. Finally, it helps the company develop action items in order to achieve the causal analysis and resolutions in the process of software development.