The conversion reaction of methanol over modified HZSM-5 catalysts having different amount of phosphorus was studied. Formation of olefins was largely enhanced over 5 wt% phosphorus modified catalyst, although the decrease in conversion of methanol was not significant. The selectivity to propylene was increasing with the amount of impregnated phosphorus, while the selectivity to ethylene was decreasing. The increase in the selectivity to propylene could be explained by decrease of strong acid sites and suppression of reequilibration reaction of olefins due to phosphorus modification.