It is generally believed that the porosity, permeability, wettability of reservoir varies after long term water flooding. But the researches of relative permeability curve variation of reservoir after long term water flooding were rare. A laboratory unsteady state method was used to research oil water relative permeability for cores under the approximate conditions of oil reservoirs after long term water flooding, and the results were compared with those of the conventional cores. According to this comparison if the water saturation is fixed, the water phase permeability of cores after long water drive will be higher than that of conventional cores, and the conventional cores will generally be earlier in water breakthrough and faster in water cut increase than cores after long water drive, with the cores after long water drive being higher in the water free oil recovery and the ultimate oil recovery than the conventional cores. The wettability of cores after long water drive will change after oil washing, and their remaining oil saturation will somewhat in crease due to expanding internal pore surface by the remove of solid particles during oil washing.