We have studied the effect of annealing process on the performance of photovoltaic devices based on the bulk heterojunction of poly(3-hexylthiophene) and [6,6]-phenyl-C(61) butyric acid methyl ester (P3HT/PCBM). By means of atomic force microscopy (AFM) and scanning of near-field microscopy (SNOM), we can observe the morphology evolution of the annealed P3HT/PCBM composite films. We also studied the changes of optical properties by absorption spectroscopy and the changes of composition distribution of annealed composite films. The results indicate the P3HT in the composite film gradually becomes an ordered structure with annealing. The ordered P3HT facilitates the charge transport. However, the film exhibits a large-scale (1 mu m) PCBM aggregation after annealing for an extended period of time. The disrupted bi-continous phase retards the charge transport. Thus, the device efficiency reaches the highest (2.308%) after annealing at 140 degrees C for 30 min but decreases to 0.810% after 60 min annealing. (C) 2008 Elsevier B.V. All rights reserved.