Hydrocracking of 1-methylnaphthalene (1-MN) and 2-methylnaphthalene (2-MN) catalyzed by a sulfided nickel-molybdenum/alumina catalyst was investigated under hydrogen pressure of 6 MPa (cold charge), in the temperature range of 390 to 450 degrees C. Reaction products were classified into six groups of compounds; (i) methyltetralins and their isomers, (ii) methyldecalins and their isomers, (iii) ring-opening products (monocyclic compounds), (iv) cracked products (bicyclic compounds of C-10 or smaller), (v) alkylation products (bicyclic compounds of C-12 or larger) and (vi) dimerization products (alkylbinaphthyls and their hydrogenated compounds). Ring-opening selectivity has been examined from the distribution of monocyclic products. Four isomers of C-11-alkylbenzenes were predominantly produced from 1-MN. They were (1-methylbutyl)benzene, pentylbenzene, 1-butyl-2-methylbenzene and 1-butyl-3-methylbenzene. They are considered to be produced as a result of ring-opening of 1- and 5-methyltetralin, Four isomers of C-11-alkylbenzenes were also produced from 2-MN. These were (2-methylbutyl)benzene, (3-methylbutyl)benzene, 1-butyl-3-methylbenzene and 1-butyl-4-methylbenzene. They are considered to be produced from 2- and 6-methyltetralin, In the 1-MN reaction, the bond which is close to the methyl group is selectively cleaved. However, the 2-MN results indicated that it is more difficult to open the ring at the bond closer to the methyl group.