The effect of thermal annealing on the device characteristics of the bulk heterojunction poly(3-hexylthiophene) (P3HT):fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) solar cells was investigated. Absorption spectra showed that the absorption intensity was highest for the P3HT:PCBM layer with a LiF layer annealed for 5min. Atomic force microscopy images showed that the P3HT:PCBM layer with a LiF layer annealed for 5min had the uniform surface morphology. The fabricated organic photovoltaic (OPV) cells annealed for 5min showed the enhanced power conversion efficiency of 3.87% than that of the OPV cells annealed for 3 or 10min.