This study aims to compare between usage of Nano-particles and ordinary-particles in enhancement of subgrade properties. To attain this purpose, Nano-magnesium oxide and ordinary-magnesium oxide were selected as modern alternatives in improvement of construction materials. A selected soil supplied from Baghdad city was treated with different dosages of the proposed additives to investigate the changes in engineering properties of the selected soil. The study involved laboratory tests to compare the properties of untreated and treated soils with Nano-additive and ordinary-additive. Basically, this study adopted California bearing ratio, swelling ratio, and repeated triaxial compression tests. The results revealed the preponderance of Nano additive over the ordinary one in enhancement of subgrade properties. Afterward, chemical tests and tests by x-ray were implemented to reveal the chemical reaction that changed the properties of subgrade. These tests revealed high increase in alkalinity of treated samples. In addition, the results revealed effective chemical reaction generated magnesium silicate hydrate which produce strong bond among the particles. Moreover, chemical tests results exhibited that using Nano additive lead to more intense chemical reaction compared to ordinary one. This behavior may be attributed to the unique properties of Nano particles with super fineness and large surface area.