Variation is the fundamental cause of poor quality. In order to reduce variation and seek quality improvement opportunities, it is important to understand how variation is added and transmitted across the processes of the manufacturing system. In this paper, we introduce a model for variation transmission and give the method of detecting variation sources in a series connection manufacturing processes. Finally, an example of the application of this method is given. Variation is the fundamental cause of poor quality. In order to reduce variation and seek quality improvement opportunities, it is important to understand how variation is added and transmitted across the processes of the manufacturing system. In this paper, we introduce a model for variation transmission and give the method of detecting variation sources in a series connection manufacturing processes. Finally, an example of the application of this method is given.