Based on pseudo-steady-state approach, the effect of particle size and porosity on conversion characteristics of single coal char particle moving in high temperature O-2/CO2 atmosphere was studied to analyze the particle conversion mechanism. The particle size and porosity were variable in the range of 0.1-1.0 mm and 0-0.4 respectively. The particle structure model is developed referring to experimental coal char particle pore structure. The Navier-Stokes equations were coupled with energy and species conservation equations to solve the problem. Stefan flow, Maxwell-Stefan equation and Soret effect were considered for mass transport. Water-gas-shift reaction, CO oxidation reaction and four heterogeneous reactions were taken into account. The reacting particles moving in different oxygen concentration were modeled for validation against experimental results of literatures. The results show that with the decrease of particle size, the reactive zone expands relative to the particle but shrinks relative to the space and attaches to the particle surface because of the low Reynolds number and the small pore size. The particle with high porosity is more sensitive to the change of the ambient oxygen concentration due to the large contact surface with gas flow. Additionally, the detaching of the flame sheet and the endothermic Boudouard reaction are two significant factors affecting the surface temperature. The insufficient supply of oxidant causes the decrease of the specific carbon consumption rate along with the enhancing of the particle size and porosity. (C) 2017 Elsevier Ltd. All rights reserved.