The effects of surface free energy of substrates on the soiling and on the detergency of the oily soil were studied. The surface tension consisted of dispersion force and polar force components of substrate, oily soil and surfactant solutions were calculated by extended Fowkes` equation. From these values, work of adhesion(W_a), work of detergency(W_D), and residual work of detergency(W_D,R) were calculated. The correlations between these theoretical values of the works and detergency were discussed. MAA grafted PET film was used as substrate, triolein as oily soil and nonylphenol polyoxyethylene ether(NPE) having various mole numbers of oxyethylene adducts and dodecylbenzene sulfonate(DBS) as surfactants. Detergency was estimated by means of radioactive tracer method using C_14-triolein. The results showed that W_a, was decreased with the increase of surface free energy of substrate. In water, W_D and W_D,R were decreased and detergency of tiolein was increased with the increase of surface free energy of substrate. In surfactant solutions, the lower the surface free energy of substrate and the lower oxyethylene adducts of NPE were the more effective on detergency. The detergency of DBS solution was the lowest in the case of ungrafted PET film, but even small increase in surface free energy by grafting showed much increase in detergency.