Abstract: To evaluate the time effect on mechanical properties of orthodontic arch wires. Methods30 nonextraction patients treated in the department of orthodontics in Shandong University in the same time period were enrolled in this study. The threepoint bending test was used to measure the strain and stress of arch wires at 37?℃. Oneway ANOVA and q test were used to compare the lag values and platform stresses. ResultsThe platform stress of the control group was 1.54±0.05N and the lag value was 0.47±0.05N. The platform stress of wires with a 4week usage was 1.35±0.03N and the lag value was 0.61±0.03N. The platform stress of wires used for 8 weeks was 1.38±0.07N and the lag value was 0.46±0.07N. The platform stress of wires used for 12 weeks was 1.02±0.04N and the lag value was 0.34±0.04N. By oneway ANOVA and q test, there were significant differences between each group（P＜0.05）. ConclusionWith the increment of use time, the properties of arch wires are attenuated, but their elasticity is still good.

Key words: NiTi arch wire, Mechanical properties, Superelasticity