Two types of rectangular orthodontic archwires; NiTiTM and 40oCuNiTi, were heat treated by Direct Electric Resistance Heat Treatment (DERHT) using different electric currents for 4 s. Their mechanical properties were then evaluated by micro hardness and three-point bending tests. After applying 4.5-5.5 A current, the hardness of NiTiTM increased with the increased current, whereas the change in hardness of 40oCuNiTi was slight. When 6 A current was applied, the hardness of the midspan of both wires significantly decreased. From the three-point bending test, unloading forces of NiTiTM increased after treating with 5.5 A current, while those of 40oCuNiTi decreased. However, both specimens lost their superelasticity when applied with 6 A current. In conclusion, after DERHT, various changes in mechanical properties can be noted in the different types of nickel titanium archwire.