Several reports have pointed out that endodontically treated teeth can lack strength, and that the teeth can be reinforced using posts. However, it has not been clear how to select posts that meet the needs of most clinical situations, particularly in terms of the post diameter, which has a major influence on the occurrence of root fracture. The purpose of this study was to analyze the stress distributions of posts of various diameters during masticatory loads using a finite element method. A 3-dimensional (3D) finite element model of a lower first premolar was developed. We used the image software Geomagic Studio (3D Digital 2002; Geomagic, Research Triangle Park, NC, USA) to reduce the post diameter by 6 ratios to a root diameter of 20, 30, 40, 50, 60, and 80% and then individually implemented them into the root of a tooth. A chewing static force of 100 N was applied as a 45° diagonal load on the buccal cusp tip, and the σ(von Mises) and σ(max) stresses were calculated. Analysis of the σ(von Mises) values revealed that the stresses were concentrated in the middle 1/3 of both the post and the root surface for all models, as were the σ(max) values. The results also indicated that when the diameter of the post was 50% of that of the root, the stress distributions of the post and the root surface were most favorable. In conclusion, the clinical implications of the results will need to be further studied and discussed.