Abstract: In order to reducing olefin fraction in FCC gasoline, USY zeolite, the active component of FCC catalyst, was modified by impregnating or ion-exchanging chromium. Acidity and cracking activity of modified USY zeolite were studied by IR and MAT. It was found that both amount and strength of Bronsted acid site on the surface of modified USY zeolite were increased obviously, but the Lewis acid site was almost unchanged. The hydrogen-transfer activity of modified USY zeolite was independent of the ways of loading chromium, but the isomerization activity of USY zoilite was dependent of the loading methods. The impregnated Cr-USY was better than ion-exchanged Cr-USY. The activities for cracking, hydrogen-transfer and isomerization reaction of modified USY zeolite were increased first, and then decreased with increasing of chromium amount. The cracking catalyst, USY zeolite with 0.01% chromium as the active component, showed higher activities for cracking, hydrogen-transfer and isomerization reaction. The olefin content in cracking gasoline was decreased evidently, and the isomeric alkane, cycloalkane and aromatic hydrocarbon were increased. The extent of olefin decline was high to 28 %, but the contents of isomeric alkane, cycloalkane and aromatic hydrocarbon were enhanced in evidence, they were 6.8 %, 20 % and 27.6 % separately comparing to the USY zeolite cracking catalyst. The decreasing of olefin and increasing of cycloalkane and aromatic hydrocarbon improved the octane number of cracking gasoline. It was indicate that the chromium modified USY zeolite was favorable to the olefin-reduce cracking catalysts.