Experimental investigations were carried out with mainly a Mogensen-Sizer, compared with test screening and additionally laser diffraction and light extinction, in order to check the qualification for on-line determination of particle size distribution under the specific conditions of feed milling. The different components of compound feed, the degree of milling, the differences in measured particle characteristics and the possibility of sample dispersion affect the comparability of the results. The results show that laser diffraction is a many-sided method with accurate recording of the distribution. The modified Mogensen-Sizer can be a robust low-price alternative if the control of selected distribution parameters is sufficient.