Micro-size tungsten particles have been prepared by radio-frequency (RF) thermal plasma reactor. SEM images show that spheroidization ratio of small particles is obviously lower than that of big particles. Numerical model has been founded to simulate the spheroidization system to explain this phenomenon based on FLEUNT software. The calculation results indicate that small particles are easy to diffuse and 'back-mix', which will urge small particles to escape from the high temperature area, while big particles are flowing straightly through the high temperature area, as a result that small particles cannot absorb enough heat and cannot be spheroidized well. The forces of diffusion and 'back-mixing' are each radial velocity and axial velocity. With some calculations based on the change of each gas flow, it can be found that appropriate combinations of gas flow can improve the spheroidization ratio of small particles. (C) 2016 Elsevier B.V. All rights reserved.