A high boiling fraction of straight run gas oil, which contained 4,6-DMDBT by addition but free from dimethylbiphenyl and dimethylcyclohexylbenzene of its HDS products, was desulfurized over commercial catalysts to find principal products of 4,6-DMDBT. Thus, HDS of 4,6-DMDBT in the high boiling fraction of real feed was proved to proceed principally through hydrogenative route. During the HDS of the fraction, 4,6-DMDBT was found to be produced from trimethyldibenzothiophenes. This activity depended very much on the acidity of the catalyst.