In this paper, the effects of optical interference and annealing on the performance of P3HT:PCBM based organic solar cells are studied in detail. Due to the optical interference effect, short circuit current density (JSC) shows obvious oscillatory behaviour with the variation of active layer thickness. With the help of the simulated results, the devices are optimized around the first two optical interference peaks. It is found that the optimized thicknesses are 80 and 208 nm. The study on the effect of annealing on the performance indicates that post-annealing is more favourable than pre-annealing. Based on post-annealing, different annealing temperatures are tested. The optimized annealing condition is 160℃ for 10 min in a nitrogen atmosphere. The device shows that the open circuit voltage VOC achieves about 0.65V and the power conversion efficiency is as high as 4.0 % around the second interference peak.