Using molybdenum trioxide (MoO3 ) as anode buffer layer, organic photovoltaic (OPV) cells with a structure of ITO/MoO3 /P3HT/C60 /Bphen/Ag was studied. The effect of thickness of MoO3 on electrical characteristics of the device was systematically investigated. From the conventional equivalent circuit model, the series resistance of cell was calculated. Also, the absorption spectrum was measured by a spectrophotometer. The results indicate that at the optimized thickness of MoO3 , the short circuit current, open circuit voltage and fill factor were all increased. The MoO3 layer plays an important role in forming good interface contact between anode and donor, which decreases the series resistance and improves the carrier transfer and collection efficiencies. Moreover, MoO3 has high optical transparency and can not decrease the photon absorption efficiency.