The effects of the vacancy-type defects in the graphene sheet (GR) and volume fraction ( \({\text{v}}_{\text{f}}\) ) of the GR on Young’s and shear moduli of polylactic acid (PLA) nanocomposite strengthened by the GR (GR/PLA) are investigated. The molecular dynamic (MD) method is implemented and stress–strain evolutions are extracted to explore elastic constants. The simulations demonstrate that adding the defect-free and defective GR in the PLA leads to a vast improvement in tensile and shear moduli. In every \({\text{v}}_{\text{f}}\) of the GR, the defective GR/PLA under tensile loadings compared to the defect-free one can endure smaller stress and deformation at the breaking point. Likewise, the bearable stress of the defective GR/PLA subjected to longitudinal shearing is lower than the maximum stress obtained from the defect-free GR/PLA. In any \({\text{v}}_{\text{f}}\) of the GR, as the rate of the defects rises, the defective GR/PLA is capable of withstanding a smaller quantity of ultimate stress. However, the variation of the ultimate deformation of defective nanocomposites with the increase of the defect content does not pursue a determined trend, mainly because it is heavily dependent on the distribution pattern and location of defects. Young’s and shear moduli of the GR/PLA experience a downward trend with increasing the degree of the defect. In a desired defect percentage, the stiffness and rigidity of the nanocomposites become larger by choosing a higher GR’s \({\text{v}}_{\text{f}}\) .
