Abstract: Original Cam-clay model, modified Cam-clay model and a newunified constitutive model for both clay and sand are widely used inboth research and engineering applications. Within the classicelastoplastic theory, those constitutive models for geomaterials arededuced afresh in this paper. Comparisons between those constitutive modelswithin the classic elastoplastic theory and thepractical geotechnical engineering are made. The relationships betweenthe corresponding variables and parameters are obtained.