Abstract: In this paper a new generation of methodology of CAD is used to develop the design and CAD system of modular robots.It is aimed at providing the guideline and framework of intelligent software for solving design of modular robots in flexible assembly line.Taking the modular robot design as a breakthrough,a task oriented and case based design methodology is applied in the conceptual design of mechanical design.The rapid development of standard modules and modular robots is introduced. According to the characteristics of the conceptual design of modular robots and mechanism of case based reasoning in AI,a new methodology and CAD system,and a top down case based reasoning flow are suggested. The end user oriented descriptions of robot tasks and working environments are also introduced in this paper.