Hydrogenation of phenanthrene (PHE) and the mechanism of catalyst deactivation in the presence of dibenzothiophene (DBT) and acridine (ACR) using Ni-Mo/Al_2O_3 and Pt-Pd/Al_2O_3 were investigated. In the Ni-Mo/Al_2O_3, the conversion of PHE slightly increased with decreasing DBT concentration. When DBT was added again after 0wt% of DBT, the activity became lower than the initial activity. In contrast, no deactivation was observed with Pt-Pd/Al_2O_3 On using Ni-Mo/Al_2O_3, octahydrophenanthrene was a major product, while with the Pt-Pd/Al_2O_3 perhydrophenanthrene became a major product with low DBT concentration. With the addition of acridine, the conversion of PHE decreased and the initial activities were not recovered for both catalysts ever after acridine was removed.