Shirasu is a volcamic soil which is widely distributed in South Kyushu in Japan. It contains non plastic fines which was created by its fragment of coarse particles. In this study, a series of bender element tests on Shirasu was performed with varying its fines content. As a result, it was observed that the shear modulus of Shiarasu was decreased with increasing the fines content although void ratio decreased as fines content increased. However, it was found that there is a unique relationship between shear moduli of Shirasu and the equivalent granular void ratio, in which the proper contribution factor b for fines was assumed. Based on the findings in the study, an empirical formula to express the shear moduls of Shirasu with various fines content was developed as the function of the fines content, the equivalent granular void ratio and the effective confining stress.