In this study the effect of nano-silica on cement-silica fume paste, mortar and concrete is studied at three different water-binder ratios: 0.20, 0.25 and 0.30. By adopting a systematic procedure most suitable nanosilica was selected from a set of commercially available nano-silicas. Most effective superplasticiser was selected from a set of super plasticisers and used in the paste, mortar and concrete to achieve maximum dispersion of the particles in the cementitious system so as to attain maximum flow. The compressive strength of these systems was tested at curing ages of 3,7,14, 28 and 56 days. The microstructure of the same system was also studied using the various instruments such as X-ray Diffraction (XRD), Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM). The results obtained from the specimens were compared with controlled specimens. As expected, the results showed that nano-silica increased the compressive strength as well improved the microstructure due to pozzolanic as well as filler action. Copyright © 2013 ACC Limited.