Hydrocracking of cis and trans-Decalin was investigated in the presence of Ru/NiO-ZrO_2-Al_2O_3, Ru/ZnO-ZrO_2-Al_2O_3 and Ru/Fe_2O_3-ZrO_2-Al_2O_3 as catalysts by a high pressure DTA techniques. The Ru/NiO-ZrO_2-Al_2O_3 did not show effective activity for the nuclear hydrocracking of the decalins. On the contrary the Ru/Fe_2O_3-ZrO_2-Al_2O_3 showed a good activity for the nuclear hydrocracking. However, the main products were alkylated cyclohexanes. Any of the catalysts did not show sufficient activity to form paraffinic hydrocarbons. When initial pressure of hydrogen was reduced from 10MPa to 1MPa, the dehydrogenation activity of the decalins was predominant and tetralin was mainly formed. Based on these results, a role of ruthenium is concluded to be the promoter of the hydrocracking of decalins.