To clarify the effect of coal particle sizes on gasification performance of an advanced two-stage entrained-flow coal gasifier for IGCC (integrated gasification combined cycle) application, a comprehensive three-dimensional numerical model is established by incorporating the shrinking core model combined with the Langmuir-Hinshelwood kinetic rate expression, which considers the inhibitory effect of CO on char-CO 2 reaction. The flow, temperature, and species distributions were obtained, and the results are consistent with the operating data. Results show that the helical flow and eight recirculation zones in the gasifier improve carbon conversion efficiency through extending the residence time of coal particles. Slow devolatilization of large particles caused by slow heating retards volatiles combustion, and thus char combustion and gasification. As a result, less char gasification and higher gas temperature appear in the injection region of the first stage. Higher inertia of larger particles produces higher slip velocity, which enhances heat transfer and mass diffusion to char particles and increases char consumption rate in diffusionlimited regions. The regions of high inner wall temperature spread from locations around burners to the whole inner wall of the injection and bottom regions in the first stage with increase of coal particle sizes.