This paper addresses a systematic and scientific tolerance design method-six sigma tolerance design. First a specifically quantified goal is determined for tolerance design with an assembly case analysis to show how six sigma is designed into the product. The design and optimized flow is put forward. This method leads to a set of design principles and practices which are embodied in the thinking of concurrent engineering (CE). The philosophy underlying design for manufacturability (DFM) finds expression in CE. Six sigma tolerance design provides a way to achieve six sigma quality and beyond. It is the result of requirements for higher quality product at lower costs.