A CAD-integrated total design system for MEMS is developed which can perform analysis and design for mechanical performance of a MEMS structure. The software works in a parametric CAD platform and makes users to do from CAD modeling and analysis to design optimization. Basic philosophy is to assure robustness, versatility and user friendliness. To satisfy these requirements; 1) Design variables are selectable directly form CAD model, 2) Commercial codes are utilized as many as available, and 3) Design sensitivity analysis must be simple and robust. Commercial finite element codes and some newly developed modules are integrated in the system for analysis. For design sensitivity analysis, two approaches were implemented: finite difference method and the Taguchi method. The approximate methods adopted seem to be simple and robust, which can be applied to design of complex practical structures. The design sensitivity analysis by finite difference method, with nonlinear programming and trade-off study, gives satisfactory results. The Taguchi method module is integrated for robust optimal design of MEMS structure. Although it is not meant to find the exact optimum point, it is applicable to practical problems where performance characteristics are hard to evaluate, since this does not require any derivative information. Two examples are taken to examine performance of the developed design tool and proposed methodology. It relieves much of the difficulties often met in conventional design works and has shown practicability for structural design of MEMS.