Abstract: Gas chromatography/mass spectrometry (GC/MS) and gas chromatography/field ionization time of flight mass spectrometry (GC/FI - TOF MS) were used to analyze the molecular compositions of hydrocracking products. The composition properties of products and the changes of composition with conversion depth were investigated. The results show that the carbon number of normal paraffin is mainly between C18 and C40. With the conversion depth increasing, the iso-paraffin increases rapidly and the normal paraffin has little change. The contents and compositions of alkanes in the feedstocks have a significant influence on the BMCI value and cold- flow properties of the unconverted oil. The cyclo-paraffin hydrocarbon compounds in the whole distillate products, the mass fraction of monocycloalkane with the carbon number from C5 to C13 is the highest, compared with other cycloalkanes. One to three-ring cycloalkanes with carbon number of side-chain above 7 undergo an alkyl chain cracking and tend to be the cycloalkanes with lower side-chain carbon number. Aromatic hydrocarbons mainly distribute in the fractions below 350 ℃with alkylbenzene, indane and tetralone as main configulations, but little compounds exist in the form of benzene.