Protonated ZSM-5 zeolites (HZSM-5(M), M = Al, Ga, and Fe) having different acid strengths were prepared under various synthesis conditions, and the effect of acidity of the ZSM-5 zeolite on the conversion of ethanol to propylene was investigated in detail. HZSM-5(Ga) and HZSM-5(Al) showed high propylene yields. However, HZSM-5(Fe) yielded ethylene instead of propylene, i.e., only dehydration of ethanol occurred on HZSM-5(Fe). These results indicate that the acid strength is a crucial factor for the selective production of propylene. In other words, for successful selective conversion of ethanol to propylene, HZSM-5(M) zeolites must contain acid sites with an acid strength that is more than that of HZSM-5(Ga). It was also found that phosphorous-modified HZSM-5(Ga) zeolite exhibited an improved propylene yield relative to the unmodified zeolite and that the modification considerably enhanced the catalytic stability because of the suppression of both elimination of framework gallium and coke deposition.