Background. We analyzed the mechanical environment of the supraspinatus tendon using a three-dimensional finite element model with the software programs MENTAT and MARC. Methods. The supraspinatus tendon that attaches to the superior facet was extracted and modeled. The geometric shape of the humeral head was determined from computed tomography images, and the shape of the supraspinatus tendon was determined from magnetic resonance images of the shoulder at 0° of abduction in a healthy 27-year-old man. The distal portion of the humeral head was fixed, and 10 N of tensile force was applied to the proximal end of the tendon. The tensile stress was calculated. Results. The tensile stress was 1.8 MPa for the bursal side and 15.0 MPa for the articular side of the anterior portion of the supraspinatus tendon. The intensity was 0 MPa for the bursal side and 4.5 MPa for the articular side of the middle portion of the tendon. The intensity was 0.1 MPa for the bursal side and 5.2 MPa for the posterior edge of the tendon. Conclusions. Based on the three-dimensional finite element method, the maximal tensile stress was observed on the articular side of the anterior edge of the supraspinatus tendon. Our result may explain the frequent occurrence of rotator cuff tears at this site.