The influence of particle size on the mechanical behavior of calcareous sand, which was obtained from the seabed in South China Sea, was studied. A series of triaxial compression tests were carried out on samples with different particle size distributions, which the maximum diameters of particles are 2 mm and 20 mm respectively. The results show that calcareous sand presents stress-strain soften behavior. The shear stress increase with particle size. With the increase of shear strain, the volumetric strain changes from contraction to dilatation. The dilatation tendency decrease with increasing particle size and effective confining pressure. Samples with coarse grains (≤20 mm) shows completely compression under high effective confining pressure (800 kPa). The peak state friction angle and critical state friction angle were also influenced by particle size. There is unique critical state line in p^'-q plane no matter particle size.