Abstract: A dispersion system of graphene oxides (GO) nanosheets was prepared by oxidization and ultrasonic treatment. The toughening effect and mechanism of GO nanosheets on cement matrix composites were investigated. GO nanosheets structure was characterized by EDS, FTIR, XRD, SEM and AFM. The results indicate that the oxygen content of GO is 32.3wt%, the thickness of GO nanosheets is about 6 nm and there are many hydroxyl, carboxyl and sulfonic groups on the GO surface. The results of SEM images and mechanical strength indicate that GO can prompt the formation of flower-like hydration crystals and increase the tensile, flexural and compressive strength of the corresponding mortar 65.5%, 60.7%, 38.9% compared with the control samples, respectively, when GO dosage is 0.03wt%. The templating regulation mechanism of GO nanosheets on cement hydration crystals is proposed and the formation of the flower-like hydration crystals is revealed.