The driving force of densification has traditionally been modeled on the basis of local curvature changes between powder particle pairs. Extension of particle pair analysis to powder compacts involving billions of particles has not been successful because of the geometric difference between the two cases. In this paper, a densification stress model for grain boundary and lattice diffusion controlled densification is developed on the basis of a powder compact＇s thermodynamics and the internal surface area evolution. For compacts with a constant grain size, the model predicts that the densification stress increases as a function of relative density, which is in agreement with experimental trends. With grain growth, the densification stress becomes relatively constant throughout the intermediate stage of densification, in agreement with experimental data in the literature. Comparison of densification rate data with densification rate model employing the developed densification stress relation also gives good functional agreement. These agreements indicate that modelling densification stress and densification rate on the basis of internal surface area captures the essential physics of powder compact densification.