Abstract: The microstructure controls the physical and mechanical properties of loess and further affects engineering stability. In this paper,we quantitatively investigated the three-dimensional particle structure characteristics of loess from Jingyang,Shaanxi province through X-ray computed tomography(CT)experiment and Avizo software. We established the numerical simulation model of loess based on the results of triaxial shear test,and analyzed the influences of particle size,shape and arrangement on the shear behaviors of loess by using PFC software. The results show that the equivalent diameter and sphericity distribution of loess particles obey the Weibull and Gamma probability distribution function,respectively. The loess sample has a transversely isotropic arrangement,and the maximum Feret diameters of most particles are nearly parallel to bedding direction. Particle size distribution and particle sphericity mainly affect strength characteristics,and have little effect on the failure mode of loess sample. As particle size distribution becomes more uniform,the shear strength of sample increases,and strain hardening phenomenon becomes more obvious. The shear strength of samples with elliptic particles reduces with decreasing particle sphericity. Particle arrangement has a noticeable impact on the shear behaviors of sample. The sample shows obvious stress drop and develops brittle shear failure when the maximum Feret diameters of most particles are parallel to the axial loading direction. The sample has lower shear strength and develops plastic shearing band when the maximum Feret diameters of most particles are perpendicular to the axial loading direction.