In order to improve the P3HT/PCBM polymer photovoltaic devices, C-70-PCBM ([6,6]-phenyl C-71-butyric acid methyl ester) was employed instead of C-60-PCBM ([6,6]-phenyl C-61-butyric acid methyl ester). C-70-PCBM showed significant increase of visible region light absorption to enhance the photocurrent. To optimize the device performance, we changed the device fabrication conditions such as atmosphere, solvents, post-annealing time, active layer materials composition, and thickness. The best power conversion efficiency of 4.7% was achieved under AM 1.5G, 75mA/cm(2) condition.