The purpose of this work is to present a new model describing the evolution of' powder porosity versus applied pressure during uniaxial compression. This model is based on quasi-chemical treatments and population balances. It is generally admitted that compression is composed of four steps: a first reorganization of the initial particles, a fragmentation of some initial particles, a second rearrangement of the initial and secondary particle mixture and a plastic deformation stage. So the model is constructed around these four steps. Experimental results on eighteen different powders show a fair agreement with theoretical results and material physical constant values. The use of this model can allow classifying the reorganization, fragmentation and plastic deformation abilities of the products and it will be useful in order to optimize the elaboration process.