Abstract: The synthesis of decalin by hydrogenation of naphthalene was studied using sulfided unsupported NiMoW catalyst in a continuous flow high-pressure micro-reactor. The catalyst was characterized by XRD and HRTEM. The characterization results show that the characteristic peaks of Ni3S2 and MoS2/WS2 exist on the sulfided unsupported NiMoW catalyst and the length of MoS2/WS2 crystal is about 5-10 nm with 3-5 stacking layers, while the crystal of reference supported NiMoW catalyst is 3-5nm long and 1-3 layers, respectively. The conversion of naphthalene is 100% and the selectivity of decalin is 99.1% on the sulfided unsupported NiMoW catalyst under the conditions of 240℃, 6.0MPa, Sv 1.0h-1, and the selectivity of decalin on reference catalyst is only 46%. It is revealed that the conversion of naphthalene and yield of decalin increase with the increase of temperature and pressure and with the decrease of liquid hourly space velocity (LHSV) on the sulfided unsupported NiMoW catalyst.