Methanol-to-olefin (MTO) reactions were investigated over three kinds of SAPO-34 catalysts with similar acidities but different crystallite sizes to elucidate the effect of crystallite size on their catalytic performance, especially in terms of the induction period and deactivation. The prepared SAPO-34 catalysts showed similar physical and chemical properties, except for their crystallite sizes. The catalyst composed of large crystallites (similar to 7 mu m) was activated after a definite induction period and deactivated rapidly, while that composed of small crystallites (similar to 0.4 mu m) achieved complete conversion without any appreciable induction period and retained its activity for a relatively long time. This variation in catalytic performance according to the crystallite size of the SAPO-34 catalysts was attributed to the number of accessible cages near their external surface and to the diffusion limitation of the materials. Simulated conversion profiles in the MTO reactions over different crystallite sizes supported this explanation. (C) 2009 Elsevier B.V. All rights reserved.