The basic integro-differential equation describing comminution process is analytically solved by the use of a certain form of selection function and breakage function. The result agrees well with previous experience. The rate constant of the selection function is considered in relation to the stress field produced within a powder bed and to the strength of material crushed. A resulting equation expressing the selection function as a function of particle size, magnitude of force applied and some operational variables as well as material properties is compared with reference data.