Post-production annealing which improves the power conversion efficiency of the P3HT:PCBMbased organic cells are very important process. Lots of various annealing temperatures and annealing times have been used for annealing process in previous studies but not any criteria was taken as a basis for determination of annealing temperature. In this study annealing temperature was optimized for annealing time of 3 min ITO-PEDOT:PSS/P3HT:PCBM/Al was fabricated at the weight ratio of 12:8 for P3HT:PCBM. Organic solar cells were annealed at different annealing temperatures in the range of 80 degrees C-160 degrees C. The variation of external quantum efficiency (EQE) with annealing temperature was obtained. The higher EQE was obtained at 120 degrees C. Thus, 120 degrees C was determined as optimum annealing temperature. The aging experiments were performed under constant humidity, illumination and atmospheric conditions. It has been observed that the post- product annealed at 120 degrees C solar cells have more longer life time and more stable compared with the non-annealed cell and other annealed solar cells.