The aim of this work is to develop a model of the milling process of biomass. For this purpose, a classic approach is selected. The breakage probability, characterized by a material mastercurve, and breakage function are used to determine the breakage matrix. A classification function is also proposed, based on physics of the process itself and the calculation of the impact number for a particle. Knowing all these, the milling product characteristic in the steady state can be reproduced. The model has been tested for the milling of two biomasses: an herbaceous one (corn stalk/leaves) and a woody biomass (poplar) in a commercial laboratory mill. The model will reproduce quite reasonably to both cumulative and discrete particle size distributions for the two biomasses tested and for several operating conditions of the mill (rotor speed, sieve openings and feed rate), as shown validating the model results with experimental work. Other parameters as input size of the material show to be less important for the sizes essayed. The main novelty of this work is to present this milling model for biomass material, introducing also the classification system modeling and the calculation of the number of impact. (C) 2015 Elsevier B.V. All rights reserved.