Thermal hydrocracking and catalytic hydrocracking of heavy oil and model reactant have been carried out to investigate the effect of dispersed Mo catalyst on slurry-phase hydrocracking. The XRD and XPS patterns suggested that the major existence form of dispersed Mo catalyst in slurry-phase hydrocracking was MoS 2 . Experimental data revealed that the conversion of feedstock oils and model reactant increased with the presence of catalyst, while the yields of light products (gas, naphtha) and heavy products (vacuum residue, coke) decreased, the yields of diesel and vacuum gas oil increased in the meantime. Besides, the yields of aromatic hydrocarbon and naphthenic hydrocarbon in naphtha fraction decreased. Effect parameters R G (the ratio of i-C 4 H 10 yield to n-C 4 H 10 yield) and isoparaffin/n-paraffin ratio were proposed to study the reaction mechanism of slurry-phase hydrocracking, the smaller effect parameters showed that there was no carbonium ion mechanism in slurry-phase hydrocracking, which still followed the free radical mechanism, and that the isomerization ratio of products decreased with the presence of Mo catalyst.