Inverted configuration bulk heterojunction polymer solar cells based on ITO/ZnO/P3HT:PCBM/NPB/Ag were fabricated, with the donor material being poly(3-hexylthiophene)(P3HT), and the acceptor material being [6, 6]-phenyl-C60-butyric acid methyl ester(PCBM). N, N’-diphenyl-N, N’-bis(1-naphthyl)-1, 1’-biphenyl-4, 4’-diamine(NPB) thin anode buffer layers with different thicknesses, which were used to improve the performances of the devices; and the effects of NPB anode buffer were investigated. The insertion of 1 nm thick NPB improves charge collection of the device, both of the short circuit current and open circuit voltage are enhanced. When the thickness of NPB reaches 25 nm, the series resistances are significantly increased, leading to reduced device performances. Effects of different thicknesses of NPB on charge injection and collection are investigated by capacitance-voltage measurements. NPB with 1 nm thickness improves charge collection of the device but without improving charge injection, and the charge recombination mechanism is dominant if the NPB layer is too thick. NPB thin layer with appropriate thickness could be used to enhance the performances of bulk heterojunction polymer solar cells.