Reduced graphene oxide/polyimide (rGO/PI) nanocomposites are prepared via in situ random co-polycondensation of amino-modified and chemically reduced graphene oxide (rGO-NH2) with commercial diamine and dianhydrides. The chemical modification and reduction of graphene oxide (GO) endows rGO-NH2 with good solubility in organic solvents to prepare rGO/PI nanocomposites with high filler contents. rGO-NH2 is further used as a functional co-monomer to participate the polymerization of PI with full compatibility of the guest and host in molecular level. With the addition of rGO-NH2 at 2 wt% content, the thermal, mechanical properties, and hydrophobicities of rGO/PI nanocomposites are significantly enhanced with various indicators achieving or approaching the optimum. The positive or negative impacts of rGO-NH2 with various contents to the properties of the obtained nanocomposites are also illustrated incidentally from micromorphology. This effective approach provides a possibility for developing high-performance PI composites based on graphene for advanced engineering or functional materials.