In this paper, the mechanical properties of calcium silicate hydrate gel (C-S-H) are investigated with molecular dynamics (MD)simulations, in which the Tobermorite 9Å, Tobermorite 11Å, Tobermorite 14Å and Jennite crystals are treated as fundamentalmodels of the C-S-H nanostructure. The initial structures of the crystal models are firstly preprocessed and optimized, andthen are simulated in the canonical (NVT) ensemble conditions. The orthotropic elastic constants of the C-S-H nanostructureare determined and used to derive the bulk and shear moduli via the Voigt-Reuss-Hill method. The self-consistent, Mori-Tanaka, and Voigt methods are employed to compute the mechanical properties of various mixtures of C-S-H composites withdifferent porosities. The simulated results are found to be in a reasonable agreement with those obtained from nanoindentationtests, while the result obtained from the Mori-Tanaka method appears to be the closest to the experimental one. It could also be observed that the mechanical properties of C-S-H vary within a certain range as the porosity and proportionsof the components change.