The particle size of coarse silicon is a very important parameter in plasma-assisted production of nano-silicon, because the appropriate size cannot only improve nano-silicon's quality, but also reduce the cost of raw materials. In this work, simulations were formulated to investigate the effect of particles' size on silicon's vaporization process inside a radio-frequency (RF) thermal plasma reactor. By counting between particles' residence time in a high temperature region and their fully-gasified time inside the reactor, particles' motion and vaporization process can be obtained in statistics. After comparing these two parameters, the gasification ratio of coarse silicon with different sizes can be finally captured. It is shown that particles below 30 mu m all have the gasification ratio above 99.0%, which is suggested as the upper limit of coarse silicon in future experimental works. Moreover, it is also shown that gasification ratios of particles above 30 mu m are greatly reduced, illustrating that sacrificing gasification ratio to apply relatively bigger particles to prepare silicon nanopowder is inappropriate and uneconomical. (C) 2017 Elsevier B.V. All rights reserved.