Hydrocracking, hydrodenitrogenation (HDN), and hydrodesulfurization (HDS) of vacuum gas oil (VGO) were examined using Ni-, Co-, and Fe-loaded and dealuminated Y-zeolites at 300-380 degrees C under an initial hydrogen pressure of 5 MPa. The major fraction of the VGO was n-paraffins (45 wt %), and the second major fraction was alkyl-substituted 3- and 4-ring aromatics. Polar aromatics and 1- and 2-ring aromatics were minor components. Paraffins of 15-32 carbon chains and aromatic compounds of 3-4 rings were efficiently hydrocracked into a gasoline fraction over Ni-HY-A (Si/2Al = 16.0) zeolite, which gave the least yield of gaseous byproducts among the catalysts examined. Extensively dealuminated Ni-HY-B zeolite (Si/2Al = 50.0) produced a large amount of middle distillate fraction, while the yields of gas and coke that was deposited on the catalyst were markedly suppressed. Both zeolites exhibited excellent HDN and HDS activities. The Co-HY and Fe-HY zeolites showed a similar hydrocracking activity as did the Ni-HY-A zeolite, whereas the Fe-HY produced more gaseous hydrocarbons. The Co-HY and Fe-HY zeolites were inferior to the Ni-HY-A in HDS and HDN activities at 380 degrees C. The metal-free HY-A zeolite exhibited higher yields of gaseous hydrocarbons, 4-ring aromatics, coke, and hexane-insoluble fractions than did the metal-loaded catalysts. The HY-A zeolite had essentially no HDS and HDN activities, and the concentration of alkyldibenzothiophenes in the product oil was increased by condensation and transalkylation during cracking reaction. The excellent activities of the metal-loaded HY-zeolites apparently originated from the optimized acidity, as a result of dealumination, in addition to the highly dispersed metal sulfides which enhanced hydrogenation activity. The acidic strength of the catalysts was a key factor in controlling the selectivity between gasoline and gas oil production.