In　order　to　clarify　the　effect　of　blending　heavy　atmospheric　residues　(AR)　and　light　AR　on　reactivities　for　hydrodesulfurization　(HDS),　hydrodenitrogenation　(HDN),　hydrodevanadium　(HDV),　hydrodenickel　(HDNi),　hydrodeasphaltenes　(HDAsp)　and　hydrodeconradson-carbon-residue　(HDCCR)　in　hydrotreating　process,　hydrotreating　of　the　blends　from　two　ARs　at　different　ratios　was　conducted　in　a　fixed-bed　unit　at　370　degrees　C　under　a　H-2　pressure　of　13.5　MPa　over　a　set　of　commercial　HDM　and　HDS　catalysts.　The　compositions　and　properties　of　the　two　ARs　and　their　blends　as　well　as　their　hydrotreated　products　were　analyzed　and　compared　in　detail.　A　positive　synergistic　effect　of　the　blending　was　found　for　HDS,　HDN　and　HDCCR,　while　a　negative　synergistic　effect　of　the　blending　was　observed　for　HDAsp,　HDV　and　HDNi.　A　synergistic　effect　index　(eta(i),s)　was　proposed　to　describe　quantitatively　the　extent　of　the　blending　synergistic　effect.　The　observed　synergistic　effects　can　be　attributed　to　change　of　the　aromatics-to-saturates　ratio　in　the　blends.　In　addition,　composition　and　property　of　the　hydrotreated　products　from　the　blending　feedstocks　with　different　blending　ratios　were　well　estimated　on　the　basis　of　the　developed　models　with　an　average　relative　deviation　less　than　1.5　%.