In the paper, a series of bulk-heterojunction polymer solar cells using poly(3-hexyl-thiophene) (P3HT) as a donor and indene-C60 bisadduct (ICBA) as an accepter is fabricated. Effect of annealing process on the cell characteristics is investigated. It is found that the cell having a multiple temperature-step annealing can distinctly improve its photovoltaic performance. At the same time the multiple temperature-step annealing process can also significantly improve the life time of the device. The device maintains a stable performance without being encapsulated.