Heat treatment which removes internal stress enhances the mechanical properties of the orthodontic arch wire. The main purpose of this experiment was to investigate the effects of the heat treatment on the mechanical properties of the Elgiloy wire. The Elgiloy wire, 0.016″ X 0.022″ and 0.018″ X 0.025″, were heat treated in an electric oven for 5, 10 and 15 minutes at selected temperatures between 300 and 900°C. Tensile strength and load deflection rate were measured to reveal the changes of mechanical property at various con?ditions, and each specimen was observed under metallurgic microscope. Also to trace the precipitation material due to overheat treatment, a qualitative analysis was carried out with EDS system. It was found that heat treatment at a low temperature caused an increase in the tensile strength and bending resistance, and a maintenance in the fibrous in the tensile strength and bending resistance, and a maintenance in the fibrous structure of both sizes of wire. The changes observed in properties and appearance were probably due to the relief of internal stresses incurred in the metal during cold working. In both sizes of wire the tensile strength and the bending resistance continued to decrease at high temperature, and the fibrous structure continued to disappear then was not observed at 900°C. The carbide precipitation founded in grain boundary at 750°C probably was other elements carbide (Ni, Co) except Cr. The grain growth was observed at 1100°C. Optimum heat treatment for the 0.016″ X 0.022″ Elgiloy wire was 10 minutes at 500°C, and for the 0.018″ X 0.025″ Elgiloy wire it was 5 to 15 minutes at 500°C.