Abstract: One of the key issues in soil mechanics modeling is to correctly define the dilatancy function. Dense and medium dense sands dilate under drained conditions and develop negative excess pore water pressure when sheared under undrained conditions. A phase transformation line is present for dense and medium dense sand and characterizes the state from contractive to dilative. The phase transformation line is affected by the original void ratio and the effective confining pressure. In the proposed model, the state parameter which makes the phase transformation line as reference line is introduced into the stress-dilatancy equation. The plastic hardening modulus expression is used to develop a new dilatant constitutive model for sand within the general framework of bounding surface plasticity. The present model simulates, with a single set of model constants, both the contractive and the dilative responses of granular soils over a wide range of variations in stress and material internal states. The model can be used for both monotonic and cyclic loading paths under either drained or undrained conditions. It has been shown that the calculated results fit well with the test results.