Hydrotreating reaction of 4, 6-dimethyldibenzothiophene (DMDBT) and carbazole (CBZ) as heteroatom-containing petroleum model compounds and biphenyl (BP) was examined without any solvent or with a solvent such as cyclohexane or decalin to obtain basic information on the effect of addition of saturated hydrocarbon solvents on the reaction. The treatment was conducted with a presulfided Ni-Mo/Al₂O₃ catalyst under 70kg/cm² of hydrogen (at room temperature) at 300-350°C in a batch autoclave. It was observed that, irrespective of the substrates, both conversion and hydrogen consumption in the reaction increased in the order: no solvent<decalin<cyclohexane. In the treatment of DMDBT in n-paraffins in the range of C₆-C₁₂, it was also observed that the reaction was relatively more promoted as their chain lengths became shorter. These results are discussed in terms of diffusivity of the substrates in the reaction medium.