Tolerance design is interdisciplinary in nature and requires input from various stages of product development such as design, manufacturing, and assembly. The factors involved, such as equipment and labor costs, process information, and required rate of successful assembly, are highly uncertain, especially in the early design stage. This paper presents a fuzzy logic approach to model all uncertain parameters in the process for automatic optimal tolerance assignment. A method based on fuzzy logic to evaluate assembly rate is proposed, and the comparison between the fuzzy logic approach and the statistical method is discussed. It is found that using fuzzy logic, a relatively smaller sample size is needed to estimate the assembly rate with reasonable accuracy. Two cases studies are used to demonstrate the process of integrating cost, manufacturing processes, and assembly rates and their uncertainties using fuzzy logic.