A shrinking-layer model by modification of classical shrinking-core model was developed based on cell wall multi-layered structure for delignification of sugarcane bagasse with sulfuric acid (SA) catalyzed atmospheric acetic acid (AA) fractionation. The general model for overall rate of delignification was obtained and further employed to evaluate the mass transfer effects on delignification. It was found that the external diffusion could be neglected under the studied fractionation conditions, while the internal diffusion appeared to play an important role. The effective diffusion coefficient for lignin within the biomass was estimated as 6.7x10(-14)-1.6x10(-12)m(2)/s depending on temperature, solid content in the system and molecular weight of lignin. However, surface reaction also seems to be a limiting step to the overall rate of delignification. At the early stage of delignification, both internal diffusion and reaction were controlling steps, while at the late stage reaction seems to be the controlling step.