The vapor Beckmann rearrangement of cyclohexanone oxime to epsilon-caprolactam catalyzed by H-USY with different SiO2/Al2O3 ratios was studied. The catalytic performance of H-USY zeolites for the highly selective formation of epsilon-caprolactam was greatly improved by adjusting the SiO2/Al2O3 ratio and by using an appropriate diluent. It has been found that H-USY catalysts with SiO2/Al2O3 ratios of 27 and 62 exhibited excellent catalytic activity, selectivity and stability when 1-hexanol was used as diluent. The NH3-TPD results indicate that an appropriate amount of relatively weak acid sites of H-USY zeolites are effective for highly selective Beckmann rearrangement. The interaction of alcohols with H-USY zeolites has been investigated by FT-IR to explain the improving effect of 1-hexanol.