The research on improvement of electrical conductivity, mechanical strength, and thermal stability of heterocyclic aromatic polymers by reinforcement of graphene nanosheets has currently received great attention in both commercial and defence sectors. However, this improvement largely depends upon the degree of dispersion of graphene sheets and interfacial interactions in the nanocomposite. In this study, we have adopted liquid-phase exfoliation of graphene nanosheets (xGnP) in methansulfonic acid (MSA) to produce xGnP-MSA suspension. Polybenzimidazole (PBI)/xGnP nanocomposite films were fabricated via in-situ polymerization of terephthalic acid and 3,3',4, 4'-Tetraaminobiphenyl, using above xGnP-MSA solution as a reaction medium. The DC conductivity of PBI was raised by 12 orders of magnitude by reinforcement with xGnP nanosheets. The maximum tensile strength of 2076  MPa and tensile modulus of 3.41  GPa were achieved for the PSI/xGnP nanocomposite containing only 5  wt% xGnP. The improved tensile properties of the PBI/xGnP nanocomposites were ascribed to efficient reinforcement by xGnP nanosheets that allow better stress transfer between PBI-matrix and xGnP nanosheets. A good agreement between theoretical and experimental Young's modulus values was found for the prepared nanocomposites. The nanocomposite films demonstrated significant improvement in thermal stability in both N-2 and air atmospheres compared to neat PBI.