The effect of graphene oxide (GO) on mechanical strengths and durability of cement composites has been studied by preparing GO-modified cement mortars. Thermogravimetric analysis (TGA) and nuclear magnetic resonance (29Si MAS-NMR) were performed on the cement paste to research the influence of GO on the hydration process and chain structure of calcium-silicate-hydrate (C-S-H) gels. In addition, mechanical strength test, such as compressive and flexural, were performed on cement mortars. The addition of GO at 0.05 wt.% (by weight of cement) increased the compressive strength by 9.02% after 28 days. Furthermore, the flexural strength of cement mortars with 0.05 wt.% GO and superplasticizer (SP) after 7 and 28 days increased by 12.26% and 21.86%, respectively, compared with reference mortar. At the same time, the effect of GO proved to be more significant and effective in the durability tests, suggesting that GO can enhance the microstructure through hydration products to create a highly cross-linked micro structure.