Abstract: The effect of multi-wall carbon nanotubes (MWCNTs) on the nanomechanical properties of cementitious materials with sand was investigated by nanoindentation technique. The original mechanical values of the indentation modulus (M) and indentation hardness (H) under 50 GPa and 4.0 GPa were statistically analyzed. The experimental frequency plot and fitted Gaussian probability density functions (PDF) were got by deconvolution technique. The quantitative computation of the volume fraction of the low density C-S-H gel(LD C-S-H) and the high density C-S-H gel(HD C-S-H) was performed. In addition to LD C-S-H, HD C-S-H and calcium hydroxide (CH), a spurious phase is found in the hydration products. With the addition of MWCNTs, the volume fraction of LD C-S-H decreases and the volume fraction of HD C-S-H increases, the modulus of C-S-H is improved. But the volume fractions of each phase calculated from the testing results of modulus and indentation hardness are different. MWCNTs induce the formation of HD C-S-H as nanocrystals, which reveals the mechanism of how MWCNTs enhance the properties of cementitious materials.