With the reduction of available land resources, the construction of civil engineering structures is carried out on soft soil, which leads to the development of ground improvement techniques such as soil stabilization. This research is intended to study the effect of adding nano-SiO 2 on soil engineering properties, especially the shear strength and the unconfined compression strength and maximum dry unit weight, using clayey soil with low liquid limit (CL). Nano-SiO 2 was mixed with soil in three different percentages (i.e. 0.5, 0.7, 1.0 % by weight of the parent soil). The shear strength, unconfined compression strength and maximum dry unit weight of treated specimens were measured by direct shear test, unconfined compression test and compaction test. It was found that increase in nano-SiO 2 content resulted in increase in the angle of internal friction, the cohesion, the unconfined compression strength and maximum dry unit weight of the clayey soil. Based on the obtained results, in order to reach the maximum increase in strength parameters, the optimum nano-SiO 2 content occurs at 0.7 %.