The performance of cement paste as the reinforcing element in walls and transferring pipes in oil wells is of considerable importance. It is known that the mechanical properties of hardened cement can be altered under high pressure and temperature of oil well; the gravity of this change has an important role in the stability and service life of the well. This paper investigates the mechanical properties of hardened cement and GO-reinforced cementitious composites at molecular scale under the surrounding conditions similar to those of an oil well using molecular dynamics method. Results in nano scale revealed a decreasing pattern in mechanical properties of calcium silicate hydrate with increasing temperature of the oil well. However, pressure increment in the conventional range of the oil wells did not show any noticeable effect on hardened cement properties. Using GO proved to be beneficial to the calcium silicate hydrate, drastically improving its mechanical properties. Results concluded that GO nanoparticle can act as reinforcing elements in the cementitious matrix. The outcome of this research can provide more insights on the application of GO in the oil well cementitious matrix, in an ideal molecular conditions and thus passed over GO agglomeration, non-homogenous dispersion and impurities in the macro scale.