This study aimed to evaluate stress in the bone when an orthodontic mini-implant is close to the roots of adjacent teeth using finite element models (FEMs), and to investigate the causes of the high implant failure rate in the mandible. Four FEMs were used: the implant touches nothing; the implant touches the surface of the periodontal membrane; part of the screw thread is embedded in the periodontal membrane; and the implant touches the root. The effect of cortical bone thickness was evaluated using values of 1, 2 and 3 mm. Maximum stress value and stress distribution on the bone elements was determined. Maximum stress on the bone increased when the mini-implant was close to the root. When the implant touched the root, stress increased to 140 MPa or more, and bone resorption could be predicted. Stress was higher for a cortical bone thickness of 2 mm than for other thicknesses. Cortical bone 2 mm thick had a higher risk for bone resorption. A mandible with an average cortical bone thickness of 2 mm may have a higher risk for implant loosening than a maxilla with the same degree of root proximity, which may be related to the lower success rate in the mandible.