A comparative study was performed on the structure and morphology of interfacial transition zone（ITZ） between ordinary concrete and steel slag coarse aggregate concrete by means of micro hardness tester,scanning electron microscope,energy spectrum analysis and microscopic test methods. The results show that the surface of slag are porous,cement paste can tightly wrap steel slag. The ITZ width between slag and cement paste is about 40μm,slightly smaller than that of ITZ between ordinary gravel and cement paste（50μm），and the structure of ITZ is denser. Hence,it can form a strong bonding force that can improve concrete strength.