Abstract: The mechanical properties of coral sand are significantly affected by the change of gradation due to the particle breakage. A series of triaxial consolidated drained shear tests are carried out to investigate the influences of the initial gradation and confining pressure on the particle breakage and deformation properties of the coral sand. The tests result show that the stress-strain curves exhibit a strain-softening behavior for the coral sand with different gradations under low confining pressures. The volumetric strain curves show very obvious dilatancy, while greater peak strength and volumetric strain are obtained for finer particles. With the increase of confining pressure, the stress-strain curves of samples with coarse grain gradation present a strain-hardening behavior. For a particular gradation, the particle breakage of the coral sand is apparent with the increase of confining pressure, and there is a pronounced power function relationship between them. By introducing a gradation parameter β, an expression representing the relationship among the relative breakage B_r, the gradation and the confining pressure is established. Furthermore, the critical state lines for the coral sand with different gradations are parallel on the e-(p '/p_a)^ξ plane, and there is a predominant linear relationship among the intercept of the critical state lines e_Г, the initial void ratio e_ic, and the gradation parameter β. Based on this study, a critical state equation for the coral sand considering the effects of the gradation is established.