This paper presents a general model named G-WTM (General Work Transformation Matrix) for modeling inherent iterations in the design phase of a product development process. By analyzing the information flow among activities, G-WTM is able to predict the impact on the process caused by iterations in terms of time and costs. It is built on the basis of DSM (Design Structure Matrix), which has been proven as an effective tool to display a design process. A brake system design process is employed in this work as a case study to illustrate the performance of G-WTM.