The research and modeling the classification effects in a fluidized bed opposed jet mill are presented. Particle distribution is the result of the interaction between two stages of classification in this mill: gravity classification above the bed, and centrifugal classification in a mill classifier. The objective of the study is to develop a model of multi-stage classification and apply in to the process in question. The universal algorithm of constructing the matrix model of arbitrary complex classification circuit is proposed and the example of its application to modeling the classification in the mill is presented. The experimental identification of the model parameters was carried out.