To investigate the effect of particle size on mechanical properties of calcareous sand, a series of consolidated drained triaxial tests were performed on calcareous sand with different particle sizes. At a low effective confining pressure of 50 kPa, the shear peak strength increases with the increasing particle size. However, under a relatively large effective confining pressure (≥ 200 kPa), the shear peak strength decreases with the increasing particle size. Moreover, the apparent cohesion increases, and the corresponding friction angle decreases with the increase of particle size. The softening and dilatancy coefficient were proposed to evaluate the softening and dilatancy behaviour quantitatively. Calcareous sands with larger particle size show the greater strain hardening and volumetric contraction behaviour, which is more susceptible to the effective confining pressure.