The tensile residual stress that generates on surface of workpiece due to wire-EDM, causes stress corrosion cracking or season cracking, and often bring about deteriorations of some mechanical properties such as fatigue limit and impact strength. In this paper, residual stress on wire-EDM'd surface were observed concerning tempered or quenched cold work die steel, using X-ray stress measurement. The results are as follows. The value and depth of residual stress on wire-EDM'd surface depend largely on the mechanical properties of workpiece, especially the yield point. The residual stress distribution is mainly affected by electric machining conditions such as condenser capacity, charging current and other conditions such as wire tension, specific resistance of dielectric fluid, table feed rate.