Abstract: The stop-effect phenomenon on the catalytic dehydration of ethanol over HZSM-5 zeolite was observed. That is, when stopping ethanol-water feed for a period of time and restoring the feed, a dramatic increase on ethylene selectivity could be found. At different reaction conditions, it was found that the stop-effect was enhanced by increasing the stop time and reaction temperature or decreasing the feed speed. When the ethanol concentration was kept at 55%, the stop-effect could be strengthened and the high ethylene selectivity was maintained at long time. 500 h catalyst stability test showed that the stop-effect can improve the life span of the catalyst effectively. By studying the aqueous ethanol dehydration mechanism and experimental results, the preliminary conclusion is that causes of the stop-effect on the catalytic dehydration of ethanol over HZSM-5 is the accumulation of ethoxy intermediate and regeneration of empty catalytic active sites in stop operation.