Abstract: Graphene oxide (GO) was prepared by the modified Hummers method with graphite powder as the raw material. The GO/polyamic acid (PAA) precursor was synthesized by in-situ polymerization, and further imidized into GO/polyimide(PI) film by heating. The GO was characterized by XRD, Raman, FTIR and AFM, and the structure of PI films at different curing temperatures was studied. Then, the water vapor transmission rate and mechanical properties of GO/PI film were tested. The results reveal that the GO has a single layer structure with its thickness of 1.26 nm. The GO/PI composite film exhibites good water resistance performance. The water vapor transmission rate of the GO/PI composite film (thickness of the film is 50 μm) containing 0.025wt% GO is as low as 56.7 g(m²·d)^-1. The mechanical properties measurements suggest that the tensile strength and the elongation at break of the 0.025wt%GO/PI composite film are 150.8 MPa and 13.5%, respectively, increasing by 18.8% and 66.7% as compared to that of PI film (126.9 MPa and 8.1%).