The authors report the study of the dependence of the device performance of polymer solar cells based on single 50-nm heterojunction poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methylester (P3HT/PCBM) layer on annealing process. Annealing before and after cathode deposition were performed for comparison. In the case of post-annealing at 150¢XC for 60 min., the device attains a conversion efficiency of 4.9%, a fill factor of 53 %, and an open-circuit voltage of 0.67 V. These values are comparable with the highest values reported previously. The annealing process is expected to modify the network morphology of the P3HT/PCBM layer. This study demonstrates that it is possible to attain good solar cell performance with the combination of single thin active layer and post-annealing treatment. This may open up an opportunity to fabricate tandem polymer solar cells.