Abstract: In order to elucidate the characterization of side chains in aromatics of vacuum gas oil, Tahe VGO was separated into light, medium and heavy aromatic fractions according to the polarity, then the average composition of aromatics and the side chains were investigated by instruments and ruthenium ion catalyzed oxidation reaction. The results show that carbon number and average molecular weight of n-alkyl chain decrease, while the polycyclic aromatics, heteroatom aromatics increase from light to heavy aromatics. By analyzing the water phase and organic phase of ruthenium ion catalyzed oxidation reaction, it is found that about 80% of side chain of the three aromatics fractions is normal alkyl. The short side chain of C1~C7 is increased as fractions become heavier, but the amount of the side chain longer than C7 is on the contrary. In addition, most of isomerized alkyl side chains are the side chain with single methyl.