The behavior of particles during sieving was analyzed by 3-D simulation (Particle Element Method) to predict the separation size of sieves. The separation size of sieves varies according to the conditions, such as the vibration of sieving machines and the characteristics of sieved powders. To clarify the cause of this variation, the current research focused on the sieving rate and the number of trials in a unit time of sieving, xi. In the case of horizontal vibration sieving, xi increases as the centrifugal effect, K, increases. The sieving rate also increases and reaches its maximum value when K is approximately 2. On the other hand, decreases as the particle size distribution becomes coarser and the quantity of sieved powder increases. Based on these results and the microscopic observation of particle behavior during sieving, the effect of sieving conditions on was clarified. Consequently, a new method to predict the separation size is proposed.