Two kinds of NiMo/HY catalysts, with and without mesopores, were prepared starting from two NaY zeolites with different Si/Al ratios. Both catalysts possessed similar properties arising from micropores because the catalysts had similar zeolitic framework structures. For both catalysts, most of the NiMo sulfides were dispersed inside the micropores. The remaining NiMo sulfides, being relatively large particles (3-10 nm), were located on the mesopore surface for the NiMo/HY with mesopores, whereas for the NiMo/HY without mesopores, they were located on the external surface of zeolite particles. The catalytic activities were evaluated by the hydrocracking of tetralin and atmospheric residue. Both catalysts showed similar catalytic performance in the hydrocracking of tetralin, which diffused into the micropores. In the hydrocracking of atmospheric residue, however NiMo/HY with mesopores was superior to that without mesopores. These results revealed the important role of mesopores as active catalytic sites in the hydrocracking of heavy oils.