Nano technology application has added the effort to improve its properties by altering the microstructure. Nano materials, by virtue of their very small particle size can affect the concrete properties. The use of amorphous Nano silica at different cement contents 500, 600 and 700 Kg/m3 was studied to improve the compressive strength and tensile strength of high strength concrete. An experimental investigation has been carried out by replacing the cement with Nano silica of 0.5%, 1% and 1.5% by weight of cement at the different cement contents. The experimental test results showed that concrete prepared with 1% Nano silica showed a higher strength than those of the 0.5% and 1.5% Nano silica at the different cement contents and at 28 and 56 days.