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An aqueous dispersion of graphene oxide (GO) nanosheets with different oxygen content was obtained from graphite via oxidation and ultrasonication. The effects of GO nanosheets on the microstructure and mechanical properties of hardened cement paste were determined by scanning electron microscope, X-ray diffractometer and tensile/flexural/compressive strength investigations. The results indicate that GO nanosheets can regulate the formation of flower-like crystals and distinctly improve the tensile, flexural and compressive strengths of the corresponding hardened cement paste: when the oxygen content of GO was 25 . 45% and its dosage was 0 . 02% by weight of cement, the tensile strength and flexural strength increased by 197 . 2% and 184 . 5% respectively. These results suggest that GO nanosheets can significantly improve the toughness of hardened cement paste. A working mechanism is proposed by which GO may regulate the formation of hydration crystals by a template effect and thus exhibit significantly enhanced flexibility. The research results provide a potential new pathway to improve the toughness and reduce the cracking of cement composites.