In this study, nanoindentation was used to investigate the microstructures of cementitious composites containing silica fume and fly ash. With the help of scanning electron microscope, the mechanical properties (elastic modulus and hardness) of individual phases (like outer product, inner product, calcium hydroxide, remained fly ash particles, residual cement grains) in cementitious composites containing silica fume and fly ash were investigated and analyzed. Additionally, this study examined the differences between the 'C-S-H' phases in the different cementitious composites and provided an insight into the influence of mineral admixtures (silica fume and fly ash) on the properties of the 'C-S-H' phase.