This paper presents an optimum design process of micro-electro-mechanical systems (MEMS) by stochastic optimization to obtain a reliable and optimal design under the structural and fabrication uncertainty. This optimized design process shows the application of probabilistic analysis and optimization methodologies for MEMS design and fabrication. The optimal design of MEMS devices, with a MEMS microswitch as our device example, has been carried out to find the optimum load required for successful operation of the device. The objective of the optimization problem is to minimize adhesion satisfying a set of probabilistic constraints. The optimization problem, which considers both performance and structural reliability, improves yield rate greatly and is essential for successful mass production. This approach, although shown for MEMS structures, can be easily applied to conventional mechanical structures where the reliable optimized performance of the structure is important.