This work covered the application of thermally reduced graphene oxide (rGO) as a hole-transport layer (HTL) to enhance stability of normal P3HT:PCBM-based bulk-heterojunction solar device. This device showed approximately the same performance of device using PEDOT:PSS as HTL (3.14%). After 3 days, exposure of an unsealed device in 1 sun illumination, rGO device resulted in 67% degradation of performance, whereas samples fabricated with PEDOT:PSS or without HTL degraded to more than 88% in efficiency. By simulation into an equivalent circuit, impedance analysis demonstrated that this degradation was strongly depended on the buffer materials. The use of the thermally rGO as buffer material could reduce the change of resistance in both HTL and bulk layer as well as avoid the accumulation of space charges in the active film.