Abstract: Dilatancy is one of the most important characteristics for frictional granular materials, especially for geo materials. It is widely accepted that the mechanism of dilatancy could be related to the evolution of the internal topological structure within the granular system. Based on meso-structural data of granular assemblies, features of the internal topological structure evolution in the granular system can be captured, which could further help to correlate the mesoscopic topological evolution and the macroscopic deformation properties including dilatancy. In this paper, the discrete element method (DEM) was used to conduct biaxial tests on dense, medium-dense and loose frictional granular materials, respectively. According to those DEM data from macroscopic to microscopic levels, the topological mechanism for dilatancy of granular materials are investigated in terms of network parameters (e.g., coordination number and clustering coefficient) and deformation features of 3 types of mesoscopic structures induced by topological exchanges. The results show that the significant strain softening and dilatancy occur for dense granular samples under biaxial loading, which is related to the topological and geometric changes of mesoscopic structures. The medium dense sample also exhibits dilatancy features but the degree is less evident, and the loose sample only shows contractancy and strain hardening during the shearing process. The contact network could be tessellated to force loop structures with the polygon shapes, and further classified into new, lost and constant categories by considering the topological exchanges. The anisotropy and composition evolutions of three groups of force loop structures are different, and loops with larger size could exhibit higher geometrical anisotropy. Under deviatoric loads, the new loop structures are easily related to higher dilatancy, and the dilatancy mechanism of the overall granular system could be influenced by the comprehensive effects of the topological evolutions of new meso structures and geometrical evolutions of constant meso structures.