Abstract: Catalytic performances of hydrothermal dealuminated ZSM-5/Y composite zeolite in the cracking of Daqing VGO were studied in comparison with the mechanical mixture of ZSM-5 and Y zeolite. The results showed that, compared with mechanical mixture, the composite zeolite gives higher diesel and gas yields, especially higher gas olefins such as ethylene, propylene and isobutene, and lower gasoline yield. The difference of gasoline and diesel yields between the composite and mechanical mixture was elucidated from the structure of composite zeolite and shape-selective catalysis of ZSM-5. The agglomerate structure of the composite zeolite was in favor of gasoline cracking, and prevented the larger hydrocarbon molecular component from entering into it and cracking. The analysis of gas composition also supported the structural characteristic of composite zeolite.