Among several typical zeolites, H-ZSM-5 was found to be a promising catalyst for the conversion of methyl halides (CH3Cl and CH3Br) into propylene. The increase in Si/Al ratio or Na+. exchange degree in ZSM-5 increased the selectivity of propylene but decreased the conversion of methyl halides. The treatment of H-ZSM-5 with ammonium fluoride followed by calcination significantly improved its catalytic performance. With a proper concentration of fluoride, not only the propylene selectivity but also the catalyst stability could be enhanced significantly. We have demonstrated that the acidity and the pore structure are two crucial factors determining the catalytic behaviors. The weaker acidity of the fluoride-treated H-ZSM-5 suppressed the hydrogen-transfer and aromatization reactions, leading to higher selectivity to light olefins. Larger micropores with sizes of 0.73-0.78 nm, which were generated after the fluoride treatment, changed the distribution of methylbenzenes in the "hydrocarbon pool" over catalyst and contributed to higher propylene selectivity. (C) 2012 Elsevier Inc. All rights reserved.