In this work, we report an effective way to fabricate high-performance polyimide (PI) nanocomposites containing unique hybrid structure of graphene oxide and carbon nanotube. PI composites with hybrid graphene oxide (GO) and carbon nanotue (CNT) were prepared by in situ polymerization and in situ thermal reduction of GO. Both mechanical and electrical properties of the PI/GO/CNT (99:1:0.1) composites were significantly enhanced compared with those of neat PI. The electrical conductivity of PI/GO/CNT was 3.5 × 10−4 S/m which was about 109 times higher than that of neat PI. The modulus of PI/GO/CNT composites was increased up to 3.6 GPa, a 44% enhancement compared to that of neat PI, and tensile strength was increased from 71 to 104 MPa. Furthermore, the thermal properties of PI/GO/CNT composites were improved greatly compared to that of neat PI. The surface roughness of PI/GO/CNT composite films were reduced greatly compared with those of PI/GO and PI/CNT composites, which was almost the same as that of neat PI. The homogeneous dispersion of GO and CNT as well as the formation of unique hybrid structure with CNT perpendicularly adhering to graphene layers, which can embed in the polymer matrix via strong π-π interaction between carbon materials and PI matrix, were considered the main reason for the performance enhancement of PI/GO/CNT composites.