The effects of the Y zeolite crystallite size on the hydrocracking activity and middle distillate selectivity in hydrocracking of vacuum gas oil have been investigated using Ni-W catalysts with zeolite-based supports, which consisted of Y zeolite with different crystallite sizes and amorphous silica-alumina. The samples were characterized using NH3 temperature-programmed desorption, H-2 temperature-programmed reduction, Brunauer-Emmett-Teller surface area, transmission electron microscopy, and X-ray photoelectron spectroscopy techniques. The results showed that a smaller crystallite size Y zeolite had a higher surface area and more mesopores than those of larger crystallite size Y zeolites. Shorter length and slightly higher stacked layers of WS2 slabs were obtained on the catalyst with a smaller crystallite size Y zeolite, favorable to the improvement of hydrogenation activity. The catalyst containing a smaller crystallite size Y zeolite possessed higher hydrocracking activity, probably because of more available acid sites arising from a higher surface area and the more mesopores of the smaller crystallite size zeolite. Moreover, the catalyst had greater middle distillate selectivity, which could be attributed to the higher hydrogenation activity and the shorter pore channel of a smaller crystallite reducing diffusion limitation. Cat-100 presented higher hydrocracking activity and greater middle distillate selectivity than all other catalysts.