This study conducted to know about effect of welding of high strength reinforcing bar. Welding of reinforcing bars was performed with heat treatment according to carbon equivalent. AWS D1.4(Structural Welding Code - Reinforcing Steel) provides that the preheat requirements of reinforcing bar was considered according to size of reinforcing bar when carbon equivalent exceeds the standard limit. In the study, the tensile tests of reinforcing bar with preheating, post weld heat treatment, and none of heat treatment were examined. Test results were evaluated according to the criteria of yield strength, tensile strength, and elongation with KS D 3054. SD500W and SD500 reinforcing bar were used in the tests.