Abstarct: Self-compacting concrete has advantages such as faster construction of buildings, reduction of manpower, improvement of durability due to reduction of permeability, more freedom of action in the design of sections. The transition area between the cement paste and the surface of the aggregate or fibers or rebar plays an important role in the permeability, durability and strength of the concrete. In this laboratory study, Nanosilica has been used to strengthen the contact area as a highly active synthetic Pozzolan, which is a product of nanotechnology. According to the importance of the microscopic structure of this type of concrete compared to ordinary concretes and its role in the durability of concrete, it is necessary to study the microscopic structure of this type of concrete. Therefore, studies of concrete and cement paste at the nano-scale are very important for the development of new building materials and their application. In this laboratory research, the effect of Nanosilica on mechanical properties (compressive strength, tensile strength, flexural strength, modulus of elasticity and permeability and rheology (L-BOX, slump flow) of self-compacting concrete has been investigated. For this purpose, 4 mixing designs including series A, B, C and D, which contain 0, 2, 4 and 6% by weight of cement, nanosilica as an alternative to cement were tested and compared. According to the results of self-compacting concrete performance tests done in the research, it was observed that the use of Nanosilica, due to its high reactivity, reduces the performance and increases the consistency and viscosity of self-compacting concrete. Mechanical properties including compressive, flexural and tensile strengths of concrete, as the percentage of nanosilica increases to 4%, they increase and then almost decrease. According to the process of obtaining compressive strength of samples, the rate of acquisition of resistance of samples containing Nanosilica up to 7 days is the highest and decreases with increasing age of samples. According to the obtained results, it can be stated that the modulus of elasticity is sensitive to changes in the percentage of nanosilica. With increasing the percentage of nanosilica from 2 to 6%, the flexural strength of the samples has been increased. The results of water absorption test indicate a significant effect of Nanosilica in reducing the percentage of water absorption of samples compared to the control sample. Examination of the obtained results shows that the use of nanosilica can improve the condition of self-compacting concrete according to ASTM1202 standard criteria from medium or high permeability concrete to low or very low permeability concrete. The results of XRD test show that nanosilica has been effective in reducing the amount of calcium hydroxide in the structure of cement paste.