Three-dimensional finite element method was adopted to evaluate the effect of different types of cavity wall defect on stress distribution in the mandibular first premolar restored with endocrown.With the use of Mimics, Geomagic, UG and Hypermesh software, five finite element models with different types of cavity wall defect of mandibular first premolar were established (Group A:intact cavity wall;Group B:lingual cavity wall;Group C:buccal cavity wall;Group D:distal cavity wall;Group E:buccal and lingual cavity wall) .A load of 100 N was applied at the central area of the inclined plane of buccal cusp, either parallel or with an angle of 45° to the long axis of the tooth.The stress distribution in restoration and dental tissue of the five models was calculated.Results include:1) Compared to axial loading condition, the stress level in cervix of restoration, dental tissue and cement layer is much higher when the 45° load is applied;2) Under 45° loading condition, the equivalent stress peak value in dental tissue of group C is the smallest, by contrary, dental tissue of group B obtains the largest equivalent stress peak value;3) Significant improvement of stress level in cement layer was observed when dentin wall defect existed, especially for distal or mesial dentin wall defect.The buccal cavity wall plays an important role in resisting lateral force from palatal side.Defect of cavity wall makes significant improvement of stress level in cement layer, invalidation may occur under the effect of long term alternating force.