The modification of HZSM-5 zeolite was carried out by nitridation of commercial HZSM-5 zeolites with flowing pure ammonia at high temperature. The modified HZSM-5 zeolites maintained high crystallinity and high specific surface areas of precursors testified by XRD, BET and SEM analysis. Compared with parent HZSM-5, the surface strong acid sites of modified HZSM-5 zeolites decreased severely as characterized by means of NH3-TPD analysis and pyridine-IR spectroscopy shows that nitridation mainly occurred in Bronsted acid sites. The modified HZSM-5 zeolites exhibited much higher selectivity to para-diethylbenzene (DEB) and significantly enhanced stability than parent HZSM-5 when applied in the reaction of ethylbenzene ethylation with ethanol. The reduction of strong acid sites can suppress the isomerization of para-diethylbenzene and improve the para-selectivity. The effects of nitrogen contents in modified zeolites on the conversion and selectivity to p-DEB were also studied. (c) 2006 Elsevier B.V. All rights reserved.