Abstract: The effect of reaction conditions, including temperature, pressure, space velocity and hydrogen to oil ratio, on the hydrogenation of naphthalene to decalin over Ni/Al₂O₃ catalyst was investigated in a high pressure fixed bed reactor. The results indicate that the conversion of naphthalene and the selectivity to tran-decalin and cis-decalin are closely related to the reaction conditions. The ratio of tran-decalin to cis-decalin increases with an increase in the hydrogen to oil ratio and reaction temperature, but decreases with an increase in the liquid hourly space velocity (LHSV) and reaction pressure. Under a temperature 260-290℃, 5-7 MPa, a LHSV 1-1.5 h^-1, and a hydrogen to oil ratio higher than 250, the conversion of naphthalene is 100% and the selectivity to decalin is close to 100%, with a tran-decalin to cis-decalin ratio of about 4.0. Meanwhile, it was found that the sintering and/or loss of active component are the main factors that cause the deactivation of Ni/Al₂O₃ catalyst in naphthalene hydrogenation and influence the ratio of tran-decalin to cis-decalin in the products.