[en] Method fo determinin particle size distribution in material flow moving wth constant velocity is described. The method eies in transmission o narro ray of X-quanta through the flow of moving particles an detection of quanta which passed through the flow. Distribution of time intervals between quanta before and after passage through material is detected in order to improve the accuracy of measuring small size particles. The difference of time interval distributions is used to determine particle size distribution. A device for determinin particle size distribution in material flow is described