Abstract: In order to study the effects of nano-MgO on clayey soil strength, the nano-MgO was evenly mixed in the clayey soil for direct shear tests, and microscopic particles on the shearing surface were observed by use of the scanning electron microscope tests. Given the conditions that the soil water contents were 10%, 16% and 22%, nano-MgO was put into soil samples to carry out experiments with the dosages of 0, 2%, 4% and 6%. The results from direct shear tests show that when the soil water content was the same, with the increase in nano-Mgo admixture, the cohesive force of the soil samples increased, but the influences of the nano-Mgo admixture on the internal friction angle were not significant; when the nano-MgO admixture was the same, with the increase of soil water content, the cohesive force showed a tendency to increase and then to decrease, while the internal friction angle decreased gradually. The results of SEM show that the nano-MgO admixture can reduce the void ratios of the clayey soil and reinforce the cementation between soil particles, which leads to the changes in the shear strength of the clayey soil.