The effects of hydrogen addition on soot formation in C2H2/air premixed flame was investigated by numerical simulation. In order to clarify the influence of hydrogen addition on soot formation and oxidation, three different contents (0%, 20% and 40%) of hydrogen addition were performed respectively. A Monte -Carlo stochastic method was used to solve the particles dynamical model, which including particle inception, coagulation and surface reaction. Particle size distribution functions (PSDF) in different height above the burner (HAB) were obtained with different amounts of hydrogen addition into C2H2/air flame. The results showed that mole concentrations of important soot precursor species, such as C2H2 and PAHs, are inhibited by the addition of hydrogen. Thereby, the soot volume fraction was reduced. Meanwhile, the number of maximum particles was reduced when hydrogen added to the flame. Hydrogen is an effectively additive in reducing soot formation. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.