Abstract: Graphene oxide (GO) was prepared by improved Hummers method with few layers of graphene as the raw material and then grafted with maleic anhydride (MAH) to obtain MAH-GO. The MAH-GO/MBMI-E51 compo-sites were prepared with in-situ polymerization using 4,4′-diaminodiphenylmethane bismaleimide resin and bisphenol A epoxy resin (MBMI-E51) as matrix, 4′4-diaminodiphenylmethane (DDM) as the curing agent and MAH-GO as the reinforcement. The microstructure of MAH-GO was characterized and the effect of the reinforcement on the mechanical properties of composite were studied. The results show that the MAH is successfully grafted on the surface of GO through chemical bonds by FTIR and XRD, with clear lamellar structure and curls on the surface. The grafting rate is about 7.26% with chemical titration method. The microstructure of the composites indicates that the fracture morphology of the composite material has changed from straight “river-like” to “dendritic” and then to dense “dimple”, with the addition of the reinforcement. The fracture mode has realized the transition from brittle fracture to ductile fracture. The impact strength, flexural strength and flexural modulus of the composites are 24.18 kJ/m², 209 MPa and 14.15 GPa, which are 200%, 81.7% and 524% higher than those of resin matrix, when the content of MAH-GO is 1.25wt%. The mechanical properties of the composites are greatly improved.