Abstract-Nanosilica is the extremely fine silica that has been used for years as cement admixture both in the oil field and in construction and civil engineering industries. Because of its extremely fine nature and high reactivity pozzolanic material, nanosilica has been used to improve slurry impermeability and the mechanical properties of the hardened material. Using nanosilica in slurry composition, improved reological and mechanical properties of cement slurry and set cement. With the adoption of very fine silica particles in cement matrix, porosity and permeability significantly were declined 33.3% and 99% respectively and compressive strength grew from 1486 psi to 3801 psi. Finally the proper amount of nanosilica in slurry composition is presented. The novel slurry formulation with low porosity and permeability and high compressive strength, is suitable in zones where the possibility of gas migration is very high.Index Terms-nanosilica; cement class G; porosity and permeability; slurry formulation I.