Objective: This study aimed to investigate the reason for high failure rate of miniscrews during orthodontic treatment. We hypothesised that decreasing the length of the mini-screw outside the bone relative to the length inside the bone (outside/inside length ratio) and equalising it to the tooth crown-root ratio would lead to increased stability of the mini-screw against lateral load when assessed using finite element analysis. Methods:We analysed stress distribution of mini-screws in the cortical and trabecular bone and von Mises stress levels when a 2-N force was applied to the head of four miniscrews of 6, 8, 10 and 12 mm in length. The direction of the force was perpendicular to the major axis of screws. Results: Stress levels of screws in the cortical bone increased in proportion to the length of mini-screws outside the bone. The length of mini-screws inside the bone did not affect stress levels in the cortical bone. Conclusions: The results of this finite element analysis indicate that, to stabilise the mini-screw, controlling the screw length outside the bone is more important than controlling the outside/inside length ratio.