Abstract: The initial shear modulus of soil G₀ is an important parameter for predicting the ground deformation and dynamic response of many earth structures subjected to earthquake or explosion. Loess is spread all over the world and is usually in unsaturated state in the field. The principal objectives of this research are to investigate the initial shear modulus and pore-size distribution of an unsaturated loess by using the suction controlled resonant column tests and mercury intrusion porosimetry tests. The test results show that G₀ increases with the increasing suction during drying and decreases with suction during wetting. The main reason is that the amount of large pores decreases while that of small pores increases during drying. As a result, the amount of meniscus water increases, which holds soil particles more tightly. Under the same suction, the amount of small pores is larger for specimens during wetting than that during drying, which results in stronger meniscus water effect and thus G₀ during wetting is higher than that during drying.