Due to intense moisture fluctuations, expansive soil subgrades exhibit swelling and shrinkage behavior which results in the damage of lightweight structures such as pavements, single storied buildings etc. resting on them. Frequently, lime or cement is used as a stabilizer to improve the expansive soils. In addition, low calcium based stabilizers such as fly ash, grounded blast furnace slag etc. have been promoted to treat the expansive soils. In this study, an attempt has been made to study the influence of addition of fly ash on the dynamic properties of expansive soils. A class C fly ash is used to treat a moderately expansive soil having a free swell index of 50 %. A series of resonant column tests were performed to determine the small strain dynamic properties, shear modulus and damping ratio, of the untreated and fly ash treated expansive soils at different curing periods. Results indicate that with an increase in fly ash content, the shear modulus increases and damping ratio of the expansive soil decreases. In addition, with the increase in curing period there is an increase in shear modulus and decrease in damping ratio. © ASCE.