Using ultra-low permeability cores, the influences of surfactants with different interfacial tension on single phase and two phase starting pressure gradient of oil/water as well as relative permeability curves were studied. The effects of the surfactants on reducing injection pressure and enhancing oil recovery were also studied. The basic mechanisms of surfactant flooding to enhance oil recovery of low permeability reservoirs were obtained. The results show that the single phase starting pressure gradient of oil or water significantly decreases while the interfacial tension of the surfactant solution used as a displacing fluid. At the same ratio of displacing velocity, starting pressure gradient of oil and water two-phase gradually decreased, and water saturation gradually increased, as the interfacial tension decreased. The relative permeability curves of core had following characteristics: while oil/water interfacial tension decreased, the two-phase region became wider and the residual oil saturation decreased, and water relative permeability at residual oil saturation increased, and ultimate oil recovery increased. The results of the core displacement experiments show that injecting surfactant slug after the first water flooding can decrease the injection pressure significantly and enhance oil recovery to some extent. And,the lower the oil-water interfacial tension, the better the effect of enhancing oil recovery. And, the lower the oil-water interfacial tension, the better the effect of reducing injection pressure.