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The empirical expressions for predicting the small-strain shear modulus (G0) of granular soils in current engineering practice are established mainly on experimental data under isotropic stress conditions. In most geotechnical applications, however, soils are subjected to anisotropic stress conditions. The impact of stress anisotropy on G0 is a critical concern but is not yet fully understood. In this paper, we present a specifically designed experimental study to address the question. Various principal stress ratios were applied to isotropically consolidated sand specimens in a triaxial apparatus, and the elastic shear waves were generated by the bender elements installed in the apparatus such that the variations of G0 from isotropic stress states to anisotropic stress states were determined. Three quartz sands with different particle shapes were tested under a range of states in terms of void ratio, axial stress, and radial stress. The study shows that the impact of stress anisotropy is much more complicated than commonly thought. It depends on the magnitude of the stress ratio and the loading mode. A simple model that accounts for two primary mechanisms associated with the impact of stress anisotropy is proposed, and its performance is evaluated using various sources of data in the literature. [ABSTRACT FROM AUTHOR]