Fuzzy comprehensive evaluation (FCE) is a mathematical method of multi-factors decision-making, which is be used to evaluate the effects of factors on objects by application of fuzzy linear transformations.The factors important grades are the key of FCE. In this paper,the design of experiments (DOE) method is applied to determine the fuzzy set of factors important grades instead of the Delphi and expert investigation methods. And the method is applied to determine the fuzzy set of structural parameters to diesel engine body volume. The results show that the method is effective to determine the fuzzy set of factors important grades.