This paper presents a method by which the particle size distribution functions of the product in each stage of a gradual reduction milling process can be introduced. The particle size distribution functions were introduced by which the particle size distribution data of the product in each stage on several milling tests at three levels of grain conditioning were fitted to the particle size distribution function model using each milling condition as parameters by applying a least squares method. They represent the relation between the fluctuation of each milling condition in milling process and the fluctuation of organization of the particle size distribution of the product in each stage. The purpose of the introduction of them is the establishment of the interchangeable calculating method of intermediate products amount on the actual industrial processes.