Abstract: The size segregation of grains is studied with two-dimensional molecular dynamic simulations for dissipative particles under horizontal vibration.A series of segregation that induced by horizontal vibrations at different amplitudes and frequencies are reported and the relationship between these two aspects is analyzed.A standard is given to scale the speed of the segregation.It is found that an optimal amplitude exists at a given frequency.An empiristic formula is obtained for this amplitude.It shows that another segregation mechanism exists at a large amplitude and frequency,in which the big particles are thrown directly to the top by the side wall.

Key words: granular flow, size segregation, molecular dynamic simulation