Highly plastic expansive clays swell or shrink due to change in moisture content and they often have very low bearing capacity. Construction of engineering structures particularly pavements and lightweight buildings on problematic soils such as highly plastic expansive clays may create severe structural problems due to poor engineering properties of that kind of soil. This study focused on the effects of fly ash on strength characteristics of highly expansive clays which are problematic soils due to their undesirable engineering properties such as high liquid limit, plasticity index, swelling and shrinkage characteristics, and low bearing capacity. Atterberg’s limit tests and hydrometer tests were conducted to obtain consistency limits and grain-size distribution of the highly plastic expansive clay. Standard proctor tests for clay samples with various fly ash contents such as 10%, 15%, and 20% were performed to determine maximum dry densities and optimum water contents of the mixtures of clay and fly ash. The clay and fly ash mixtures at maximum dry density and optimum water content were prepared for unconfined compression and California Bearing Ratio (CBR) tests. Effects of fly ash content on the strength characteristics of a highly plastic expansive clay were investigated through unconfined compression and CBR tests. The experiment results show that fly ash is a promising additive to improve strength characteristics of highly plastic expansive clays. Addition of 20% fly ash into the clay mixture in weight results in an approximately 66% increase in unconfined compression strength and CBR values of the clay mixture.