The objective of this investigation is to study the effect of nano-silica addition on the fresh and hardened properties of ultra high strength concrete. Nano-silica with 99.8% SiO2 content and particles size of 5 to 20 nm used in concrete mixes as a replacement of silica fume content. The total percentage of silica fume and nano-silica was 20% of weight of binder content. Nano-silica was added in seven percentages (0.5%, 1.0%, 1.5%, 2%, 2.5%, 3.0%, 3.5%) of weight of cementitious materials. The properties of ultra high strength concrete was investigated through twenty seven concrete mixes using nano-silica addition with three different binder content 450, 550, and 650 kg/m3. Nanomaterials have a larger value of the ratio between surface area and volume than other similar particles in larger size , making the nanomaterials more reactive. Slump test, Compacting factor test and Ve-Be time test were used to determine the effect of nano-silica content on the workability of concrete, while compression test, indirect tensile strength test, flexural strength test, modulus of elasticity test and permeability test were carried out to determine the properties of hardened concrete. Also to compare microstructure and nanostructures of concrete with and without nano-silica, scanning electron microscopy was used. The test result showed that the addition of nano-silica as cement replacement could improve the mechanical behavior of ultra high strength concrete. The results showed that the optimum dose of nano-scilica was 2.5% by weight of cementitious materials. Using 2.5% nano-silica can produce ultra-high strength concrete with high workability without segregation. The workability of concrete decreases with the increase in nano-silica content. The use of superplasticizer was necessary in concrete mixes to improve the workability. Results indicated that nano-silica up to 2.5% nano-silica by weight could improve the mechanical and physical properties of ultra high strength concrete.