Abstract: Molecular sieve Y was modified by a combination of hydrothermal treatment, dealuminization with oxalic acid, and hydrothermal crystallization in the presence of the surfactant cetyl trimethyl ammonium bromide (CTAB). The effects of CTAB amount used on the crystallinity, unit cell constant, SiO₂/Al₂O₃ ratio, surface area and pore structure of the modified molecular sieve Y were investigated. NH₃-TPD was employed to characterize the acidity. The results showed that after modification, the crystallinity and SiO₂/Al₂O₃ ratio of the molecular sieve Y increase from 37.9% and 7.72 to 68.3% and 9.32, respectively. The unit cell constant a₀ decreases from 2.4534nm to 2.4485nm. The pore volume of mesopores increases from 0.157mL/g to over 0.180mL/g. The total acid sites are reduced after the modification. Hydrocracking of vacuum gas oil (VGO) over the modified molecular sieve revealed that the conversion and yield of middle distillates is improved by 2.42% and 4.20%, respectively, compared with that over the industrial catalyst.