Abstract: The small-strain shear modulus is an important parameter indicating soil stiffness in geotechnical engineering. In this study, the multidirectional bender element technique is used to investigate the small-strain shear modulus of calcareous sand from Persian Gulf. The effects of the coefficient of gradation, mean particle size and stress history on G_HH, G_HV and G_VH on both vertical and horizontal planes are considered. The test results show that for the calcareous sand used in this study, the stiffness anisotropy is significant, behaving as the higher G_HH than G_HV and G_VH. G_\max increases with the increase of D₅₀ and decreases with the increase of C_u. The values of G_\max at the loading stage are apparently higher than those at the unloading stage. Finally, the Hardin equation for predicting the small-strain shear modulus is modified with the participation of D₅₀, C_u and OCR, and a better prediction capability is obtained by comparing the traditional Hardin equation.