Modification of poly(acrylonitrile) membrane was carried out by heterogeneous photoinduced graft polymerization of methyl acrylate from the vapor phase. The technique of transmembrane monomer flow has proved to be effective for filling pores. The effect of preparation conditions like initiator concentration, reaction time and monomer concentration on the membrane separation was investigated. These graft modified membranes showed benzene permselectivity for benzene/cyclohexane mixture (50/50, v/v) on pervaporation.