Objective: The purpose of this study was to investigate the mechanical properties of superelastic nickel-titanium orthodontic wires under controlled stress and temperature.Methods: Three different superelastic nickel-titanium wires were examined using differential scanning calorimetry (DSC), three-point bending test and micro X-ray diffraction (micro-XRD). The three-point bending test was carried out at constant temperature (23, 37 and 60 o C) and stepwise temperature changes (37-60 o C and to 37 o C) (37-2 o C and to 37 o C). Five specimens of each wire were tested. Micro-XRD spectra were measured at the tension side of the wire when the temperature changed from 37 to 60 o C or 2 o C.Results: The load during the stepwise temperature changes (37-2 o C and to 37 o C) was consistent with that measured at a corresponding constant temperature. The micro XRD spectrum clearly showed that the austenite phase was transformed to martensite phase when the temperature is decreased from 37 to 2 o C. In a stepwise temperature change (37-60 o C and to 37 o C), the load became higher than the original load at each corresponding constant temperature. However, there was no detectable change in the micro-XRD spectrum when the temperature was increased from 37 to 60 o C.Significance: The superelastic nickel-titanium wires exhibited complicated and unexpected mechanical properties under stepwise temperature change. This study shows the possibility of qualitative analysis using micro-XRD to understand mechanical properties of these nickel-titanium wires