A new elasto-viscoplastic constitutive model (PSU-EVP) was developed for predicting a dry powder's. mechanical behavior during compaction. The PSU-EVP model it-as developed in two stages. In the first stage, an elastoplastic model was formulated by using the fundamentals of critical state theory and key elements of the modified Cam-clay model. In the second stage, the elasto-viscoplastic model was formulated using the elastoplastic model developed in the first stage and the approach the used by and given in the Adachi and Oka model. Based on these considerations, PSU-EVP model explicitly incorporates the work done in volumetric compression of pore air during powder compaction. This aspect of the PSU-EVP model is different from the modified Cam-clay model that assumes the interparticle pore space in a powder mass to be filled with fluid. This key feature of the PSU-EVP model helps in isolating the effect of entrapped pore air on the mechanical properties of powders undergoing compression. The key parameter that quantifies the effect of pore air compression is known as the voids compression ratio parameter (alpha). The parameter a varies with pressure and can take on values between 0 and 1. A compatibility parameter (phi) was also proposed to facilitate the explicit computation of the work dissipated during compaction. Incorporation of these new concepts overcomes the major limitations of existing constitutive models. Additionally, the procedure to quantify the various parameters of the PSU-EVP constitutive model is outlined.