To study the effect of screen working parameters on the motion of the oversize and on thesifting percentage of the siftage,the orthogonal design was made in terms of sifter angle, crank radius, crank speed and vibrating angle. The motion of the oversize was tested by means of ^６０Coγ tracertechniqueand the regression equation was made about the screen working parameters. The effect of the grain motion on the sifting percentage of the siftage was analysed and two sets of optimum screen working parameters were obtained.