The effects of the addition of hydrogen-donating hydrocarbons derived from petroleum were tested in a pilot plant (0.01 bbl/day) equipped with a downflow fixed bed reactor for the hydroprocessing of atmospheric residues, and the following results were obtained : (1) HHAP (partly hydrogenated product of a highly aromatic heavy fraction from petroleum) and HAP (highly aromatic heavy fraction prior to the hydrogenation to produce HHAP) were effective to reduce coke formation and hydrogen consumption. (2) A particular and effective mechanism resulting from the configurations of the additives was found to be initiated by the addition of these additives. (3) This mechanism was substantiated by detailed analyses of the products.