Here, the initial photo-degradation of encapsulated P3HT:PCBM bulk heterojunction organic solar cells is investigated. The degraded device is recovered by thermal annealing treatment. Thermally stimulated current measurements reveal that the cause of photo-degradation is carrier accumulation and that the degraded organic solar cell has two broad trap levels, of 0.71 and 0.81 eV. These traps are independent of the thickness of the photoactive layers, the mixing ratio of the photoactive materials and the cathode materials. In addition, it is confirmed that there is a close relationship between the degree of degradation and the amount of accumulated charge carriers.