A new reaction model expressing the gasification behavior of char particles is presented. This model can treat the gasification behavior of char particles that have complicated structures. The model assumes that, before the reaction, a char particle is a three-dimensional cube. The cube is composed of a large number of small, randomly arranged lattices classified as either char, ash, or macropores, depending on the proximate analysis data of the char. The diffusion of an oxidizing agent and the reaction between the char and the agent in the particle are analyzed in order to evaluate the reaction according to the reaction conditions and char particle shapes. The calculated results reveal the relationship between the char structure and the char reactivity. When the porosity is high, the reaction occurs on both an internal void surface and an external surface. The wall thickness of the char particle, determined by the size, porosity, and shape of the particle, is one of the most important factors in the diffusion rate in the particle. Therefore, the wall thickness contributes remarkably to the transition temperature between the chemical-reaction rate control regime and the pore-diffusion rate control regime. The variations in particle size, specific surface area, and reaction rate show different behavior owing to the reaction rate control regime. The fragmentation behavior also depends on the reaction rate control regime. As the diffusion controls the reaction rate, the conversion point at which the fragmentation occurs shifts to the initial stage of the reaction. In the case of small char particles, temperature has only a small effect on fragmentation behavior. (c) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.