This paper presents a life cycle assessment (LCA) method in different design process. This method can help product designers to make more environmental friendly design decisions in the design process. In this method, product’s LCA model is established in product conceptual design phase with the aid of decision matrix. Thereafter, the model is improved in the latter design phases, especially in product detail design phases. In this process, with the help of LCA software, designers can evaluate different design options and get LCA results when necessary to guide the design process. Finally, a case study of designing a coffee pot is provided in this paper to illustrate the efficiency and effectiveness of this method. The case study shows that the method performs very well in the design process. It can be used to help designers to make more environmental friendly design decisions.