Abstract: In order to improve the toughness of cement-based materials, grapheme oxide (GO) was added into cement mortar. The effect of GO on the fracture properties of cement mortar was tested by three-point bending beam fracture test, and the fracture parameters were obtained through double K fracture model. The results show that GO improves the initial fracture toughness of cement mortar. When the GO content (mass ratio to cementitious material) is 0.01%-0.07%, the initial fracture toughness of GO reinforced cement mortar is increased by 13.4%, 25.4%, 24.6% and 16.7%, respectively, compared with the control group. However, the influence of GO on the unstable fracture toughness of cement mortar is limited. The fracture energies of GO reinforced cement mortars with different GO contents are increased by 10.7%-33.3%, compared with the control group. Combined with the microscopic test, it is found that the GO influences the hydration process of cement, optimizes pore structure and promotes the generation of high stiffness hydration products. It also improves the adhesion of hydration products, and further inhibits the generation and development of micro-cracks.