Major parameter in mill modelling are Breakage Function, RTD and Selection Function that determination of them is necessary to simulation of mill product size distribution. Breakage Function Determination Software (BFDS) is developed to calculate of breakge and selection functions.This software is developed based on calculating Berube, Herbst and Fueretenau and modified H&F methods. These methods are based on percent coarser of screen, percent on screen and precent finer of screen in specific times. Fine production rate, selection function and regresion paramters are obtained of this methos and breakge function calculate of these parameters.BFDS is based on Windows OS and has a high run speed, user friendly, complete reference, search of linear and non-linear regresion parameters and selective print of results. This software is compared with OLD BREAK and Excel softwares, the results show accuracy and exactitude of this software.