We have successively fabricated inverted bulk heterojunction polymer solar cells employing ZnO and MoO3 as electron and hole selective layers, respectively. The device structure is ITO/ZnO/P3HT: PCBM/MoO3/Al. Differently from conventional polymer solar cells, ITO and Al work as electron and hole collecting electrodes in this inverted structure, respectively. We have found the optimal thickness of ZnO and MoO3 to be 100 nm and 5 nm, respectively. The highest PCE was obtained to be 3.32% under AM 1.5 illumination at 1,000W/m2, which is the highest PCE of inverted solar cells reported previously in the literature.