Soils in surface ground are usually unsaturated. Proper modeling of the characteristics ofunsaturated soil is therefore absolutely needed. However, due to its complicated mechanical behavior,establishing a constitutive model for unsaturated soil is far more difficult than that for saturated soil. In thisresearch, uniaxial compression tests were firstly conducted on unsaturated Masado (Decomposed granite)under constant degree of saturation condition to investigate the mechanical behavior of the soil at differentdegree of saturation. Then, based on the test data, the performances of previously proposed elastoplasticconstitutive model for unsaturated soil were checked carefully, taking the Bishop-type skeleton stress and thedegree of saturation as the state variables. It is found that the mechanical behavior of the unsaturated Masadocan be properly described by the proposed elastoplastic constitutive model that takes the skeleton stress andthe degree of saturation as the state variables.Keywords: Unsaturated Soil, Degree of Saturation, Constitutive Model, Uniaxial Compression Test