In reinforced concrete work, form release agents(FRA) are used to facilitate removal of forms from hardened concrete. However, it has been reported that FRA accidentally come into contact with rebar, during construction. That may lead to reduction in bond strength between rebar and concrete. Previous study demonstrated reduction in bond strength when steel reinforcement is coated with FRA. In this study mechanism behind the bond strength reduction is investigated using compressive test, x-ray diffractometer(XRD), scanning electron microscope(SEM) for finding the solution of reduction in bond strength. As concrete defiled by aqueous form release agents make weak interface and oily form release agents make oil film and space between rebar and concrete, FRA decreased the bond between rebar and concrete. Therefore, the rebar contaminated by FRA has need to consider anchorage length like epoxy coating rebar.