In the design and construction of hydraulic structures such as dams or dikes, clay normally is used as a waterproof material. However, in order to be used for these projects, the clay has to satisfy some requirements, especially the characteristics related to permeability, swelling, and shrinkage. If using natural clay, sometimes it is difficult to find the clay that sastisfied the requirements. Therefore, using treatment measures to change soil properties to suit the intended use is necessary. This paper aims to evaluate the properties of clay treated with fly ash. Some soil samples were collected from different locations in Vietnam. These samples were mixed with fly ash, with the proportions by weight increasing from 10 to 20%. The tests were conducted for some properties using the Vietnamese standard system. The results indicated that after mixing, the plastic limit, liquid limit, and plastic index of the soil decreased. The permeability of soil was also slightly decreased. In addition, the attendance of fly ash in the clay also reduced the degree of swelling and shrinkage of the soil. The study changed the fly ash content from 5 to 20%, and results demonstrated that the above parameters were also decreased with increasing fly ash content.