Objectives: Increasing the usage of fly ash in concrete helps in: i) Improving the strength of concrete with colloidal Nano silica in concrete ii) reducing the pollution of environment caused by Carbon-di-oxide emitted from cement industries. Novelty of the Study: At present, studies on Nanotechnology and Nano materials are emerging and more profound research on nanoparticles like Nano silica, Nano alumina, Nano iron oxide etc., is carried out. In this work, the impact of colloidal form of Nano size particles of silica used in concrete in the modification of the properties of concrete was studied. The concrete strength was improved due to the Nano size particles which filled the voids in between the micron size cement particle by its more specific surface area, and therefore a denser concrete was being achieved. Methods/Analysis: since two decades, numerous explorative works have been done utilizing Nano materials, such as Nano silica, Nano alumina etc., to accomplish a high strength and to increase the durability of concrete. On the principle of environmental sustainability, fly ash is used for fractional substitution of cement. The present work concentrates principally towards the addition of silica Nano particles in colloidal form with the reference concrete mix. The cement was replaced with fly ash at 30%, 50% and 75% in concrete mixture. In addition to this 1% and 2% of Nano silica was used in concrete to enhance the properties of concrete. Various tests were conducted in order to obtain the modulus of elasticity, compressive strength and split tensile strength of the improved concrete. Results show that the incorporation of the Nano silica in concrete increases the mechanical properties of concrete. © IAEME Publication.