The hydrocracking of pyrene (Py) was examined in a batch autoclave at 386 degrees C for 0-30 min under a hydrogen pressure of 5 MPa, using a Ni-supported Y-type zeolite (Ni-KY) catalyst. The hydrocracking of dihydropyrene (DPy) and hexahydropyrene (HPy) was also carried out under the same reaction conditions. In addition, a Ni-KY catalyst, whose Bronsted acid sites were eliminated by ion exchange with potassium ions, was used to investigate the behavior of Py in the hydrogenation reaction network. Over the Ni-KY zeolite catalyst, Py was hydrogenated to DPy and HPy. Reaction involving the opening of aromatic rings proceeded after Py was hydrogenated to HPy via DPy. Mono- and diaromatics, which were produced from HPy, were further cracked into C-1-C-4 hydrocarbon gases. The dehydrogenation of DPy and HPy appeared to be the rate-determining step for the case of the Ni-HY zeolite. The Ni-HY zeolite catalyst thus possessed multifunctional activities of hydrogenation, dehydrogenation, and cracking. Cracking of polyaromatics is also discussed on the basis of molecular dynamics calculation using the Insight II and Discover 3 programs. Py and HPy molecules remained outside of the Y-zeolite pore, and acenaphthene and naphthalene were able to diffuse into the micropore. However, the overall potential energy of the Y-zeolite which adsorbed polycyclic aromatic products on the outer surface was lower than that of the Y-zeolite which contained the products in the pore. Thus, it is likely that the saturated rings of Pys were opened on the outer surface of the Y-zeolite catalyst.