The effects of crystal size and mesoporosity on the performance of various synthesized ZSM-5 zeolites in catalytic dehydration of methanol to dimethyl ether (DME) reaction have been investigated. The reactions were carried out in a continuous flow fixed-bed reactor at temperatures between 180 and 320 degrees C and 1.1 bar. It was found that methanol conversion enhances by decreasing crystal size and increasing the reaction temperature. Uniform nanocrystal catalysts showed highest activity and stability for methanol dehydration to dimethyl ether in the temperature range studied. On the other hand, both the activity and selectivity of large crystals were low. The high activity and DME yield for the nanocrystals is due to smaller mass transport resistance for the Nano-ZSM-5 sample, due to the small crystal size. Furthermore, it was found that the selective dehydration of methanol to dimethyl ether on ZSM-5 catalyst is based on the product selectivity inside the pore channels; in other words, the selectivity to dimethyl ether should be 100% if there is no acid site on the external surface. From this evidence, it is concluded that in methanol dehydration to DME, the reaction activity is related to the crystal size, whereas the DME selectivity is determined by the uniformity of Al distribution, which determines acidity of external surface and low mass transport resistance of small crystal size. Therefore, a decrease in the number of acid sites on the external surface and reduction in crystal size are key ways to enhance catalytic selectivity and activity, respectively. (C) 2012 Elsevier B.V. All rights reserved.