Chemically modified USY zeolites were obtained by ammonium nitrate (AN) treatment under hydrothermal conditions. AN treatment considerably enhanced the mesopore volume of the parent steam-treated zeolite. This treatment also caused the creation of extraframework species of weak acid nature. NiMoP-based hydrocracking catalysts were prepared using the modified zeolites and evaluated in the hydrocracking of a heavy vacuum gas oil (VGO). Hydrocracking activity results indicate that, although AN treatment significantly develops mesoporosity in the parent steam-treated USY zeolite, this single treatment was not efficient to enhance the accessibility of VGO compounds to the acid sites. However, the catalysts based on AN-treated zeolites showed significantly higher middle distillates yields than the one based on a steam-treated zeolite. The creation of an amorphous phase at the mesopore walls and the external surface of the zeolite crystals was argued to support this observation. Mild Na2H2-EDTA treatment to an AN-treated zeolite showed to be beneficial to improve the hydrocracking activity. This was attributed to the enhanced access to acid sites after the removal of polymerized extraframework aluminum species (EFAl). Consequently, a modification strategy of USY zeolites that includes AN treatments is proposed to develop catalysts with enhanced middle distillates selectivity in the hydrocracking of real feedstocks.