Abstract In this study, molecular dynamics (MD) method was used to systematically investigate the effects of grain boundaries (GBs), degree of cure and temperature on the mechanical properties and thermal stability of epoxy/graphene composites. Uniaxial tensile simulations were performed for pristine graphene and graphene containing GBs reinforced epoxy composites with different degrees of cure. Results show that the yield strength and Young's modulus of epoxy/graphene composites increase with the degree of cure. The behind mechanisms can be attributed to the stronger bonding interaction between epoxy chains. The mechanical properties of epoxy/graphene composites with high degree of cure are more sensitive to GB types. The glass transition temperature ( T g ) of pristine graphene and graphene containing GBs reinforced epoxy composites presents an increasing trend with the degree of cure. The introduction of GBs can enhance the T g of epoxy/graphene composites because of the appropriate wrinkles of GBs in graphene sheets helping to block the slippage of epoxy chains. The MD predicted results of T g are in good agreement with existing experimental data and the DiBenedetto relationship can be applied using the MD parameters. Both yield strength and Young's modulus of epoxy/graphene composites decrease with temperature, which is mainly due to the weak interface interactions at high temperature and the sufficient energy of atoms at high temperature to overcome the energy barrier. GBs present diverse effects on the yield strength and Young's modulus at different temperatures. This work provides a theoretical basis for the application of epoxy composites reinforced with graphene containing GBs under different conditions. Highlights The thermomechanical properties of epoxy/graphene composites are studied. The coupling effects of GBs, cure degree, and temperature are evidenced. The existence of GBs can improve the T g of epoxy/graphene composites. MD predicted T g agrees well with experimental data using the WLF relationship.