The polyvinylidene difluoride/polyurethane(PVDF/PU) co - blend membrane was prepared using the method of wet phase transition.The formation of the micro - void of the co - blend membranes was discussed, and the relationship between the composition of the co - blend membranes and the effect of working pressure on the water flux of them was studied using membrane instrument.The morphology and structure of the surface of the co- blend membranes were also studied with SEM using the fractural theory.