In this study, the effect of nano silica (NS) on the properties of cement pastes incorporated with of fly ash cenospheres (FACs) has been investigated. A fixed amount of 1% NS was added to cement pastes containing various proportions of FAC. It was found that NS was beneficial in improving the mechanical strength of the FAC modified cement pastes. At the 7-day age, the strength increase for NS modified pastes was about 17% as compared to pastes without NS. However, the strength enhancement effect decreased with increasing age and FAC content. The dense microstructure of the NS modified pastes compensated for the higher porosity associated with FAC incorporation, leading to the reduced total porosity. With NS, there was better bonding of the FAC particles in the matrix, due to the enhanced interface. The reduced Ca(OH)2 content and partially consumed FAC spherical shells at the early age showed that NS increased the pozzolanic activity of FAC in the cement pastes.