Abstract: To investigate the effect of zeolite crystal size on the reaction of methanol to light olefins (MTO), two groups of catalysts with nearly the same acidity were obtained by modification of three kinds of HZSM-5 zeolites with Ca. Na was also used as a reference modifier to investigate the role of Ca in MTO. The crystal size and morphology of the catalysts were characterized by SEM and the acidity was measured by NH₃-TPD and Py-IR. The results showed that for MTO over HZSM-5, the selectivity to light olefins is low; moreover, the conversion of methanol as well as the selectivity to light olefins decrease quickly with time on stream. The selectivity to light olefin and the catalytic stability of HZSM-5 is increased significantly by the modification with Ca, especially at high Ca loading. Na modification may enhance the selectivity to light olefins, but the conversion of methanol over Namodified HZSM-5 still decreases quickly with the time on stream. The catalysts (either HZSM-5 or Ca/HZSM-5) with smaller crystal size also exhibit better stability. The MTO performance of HZSM-5 catalyst is affected both by the acidity and by the crystal size. Ca-modified HZSM-5 with lower acidity exhibits higher catalytic stability and higher selectivity to light olefins, which may be ascribed to that Ca is involved in the MTO cycle via both acidic and basic sites in the Ca-modified HZSM-5. Small crystal size is favorable to enhance the catalytic stability.