In recent years, geopolymers have received considerable attention because of their properties and potential applications in fire resistant materials, thermal insulation, low-tech building materials, cements and concrete, and etc. Nano silica is a material that can modify mechanical, microstructural, and structural properties of geopolymer products. In this study, nano silica was added to metakaolin-based geopolymer samples at the dosages of 0, 2, 4, and 6 % by weight. Then, mechanical properties of samples such as compressive and flexural strength were measured. The results showed that by increasing the percent of nano silica, both compressive and flexural strength increased from 62.4 to 84.1 and from 13.4 MPa to 24.2 MPa respectively. Also, effect of nano silica on microstructural and structural properties of metakaolin-based geopolymers were investigated using XRD, FTIR, and SEM micro graphs. The SEM micro graphs showed that an increase in the percent of nano silica caused an increase in the homogeneity of samples.