Effect of sintering on the reaction rate in a gas-solid system is discussed theoretically with a mathematical model, in which it is assumed that a solid pellet consists of a large number of fine particles, and that the pellet radius becomes smaller by a factor of β than that of theoretical one owing to sintering. The model clarifies quantitatively the deceleration mechanism of overall reaction rate at the later stage due to decrease of effective diffusivity of gases in the product layer. The theoretical results using this model are compared with experimental ones for the reduction of nickel oxide pellets with hydrogen. The values of β are 0.97 to 0.67 in the temperature range of 673 to 1073 K.