The purpose of this study was to assess the rate of root resorption in relation to different magnitudes of continuous force during experimental tooth movement using nickel-titanium (NiTi) alloy wire. Four force magnitudes of 0.8, 1.6, 4, and 8 g were applied to the upper first molars of 75 male Wistar rats (300-320 g, 10-week-old) for 1, 7, 14, 21, and 28 days and compared with a control group without an orthodontic appliance. Light microscopic images of the compressed periodontal ligament (PDL) were processed by computer, and the ratio of the root resorption lacuna length to root surface length without the lacuna was analysed and statistically compared using Tukey-Kramer multiple comparison honestly significant difference test. The experimental groups with 4 and 8 g force showed undermining bone resorption with degenerating tissue and marked root resorption, the 1.6 g group showed only root resorption, while the 0.8 g group was similar to the control. Comparison of the ratios showed that the 0.8 g group was similar to the control with no significant difference. The ratio on day 28 in the 1.6 g group was larger than that in the 0.8 g and control groups, while on days 14, 21, and 28, the ratios in the 4 and 8 g groups were larger than those in the control (P < 0.01); these two experimental groups showed the same significant differences. It is suggested that significant root resorption occurs when the force magnitude exceeds 1.6 g in the rat upper first molar during tipping tooth movement by continuous force, and the amount of root resorption increases with serial force magnitudes from 0.8 to 4 g.