The theoretic computation and determination of geometric tolerances are atways a difficult problem at home and abroad.In this study,based on contact mechanics principle,a model was established to simulate the mechanical behavior of contact between real feature of a part and its matching ideal feature.The maximal contact stress was proposed as a basic parameter to evaluate geometric errors.According to the ultimate design principle of upper bound with maximal contact stress,the real feature of part can be simplified to specific ideal form (named as simplified ideal form).The maximal contact stress between simplified ideal form and its matching ideal feature was calculated through finite element method.At last the geometric tolerance of part can be determined according to the variational rules of the maximal contact stress.The advantage of this method is that it can measure the effect of the geometric tolerances on the part s serviceability.The method can be used to determine form tolerances of single feature and position tolerances of associated features.It has been successfully applied to determine geometric tolerances of the combined sliding bearings of the roller bit with two radial directions and one axial direction.