Direct nitridation of titanium pellet is generally treated based on a grain model of gas-solid reaction in which chemical reaction and pore gas diffusion are taken into consideration. The morphologies of react ion fronts in the titanium pellet in various control regions are different. The progress of the reaction front of the grain of different position in the pellet has been shown in this study. Photographs of partially reacted pellet and concentration profiles of solid reactants and solid products determined by X-ray diffraction, are used to verify the model predictions.