The purpose of the study was to evaluate carbon effect on Co(Ni)-PMo/C/Al2O3 catalysts in cohydrotreating (co-HDT) of straight-run gas oil (SRGO) and sunflower oil (SO). The catalysts were prepared using 12-molybdophosphoric heteropolyacid, cobalt tartrate, and Al2O3 or C/Al2O3 (2 wt% of carbon) as a support. The solids were characterized by N-2 physisorption, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. The catalytic properties were investigated using a bench-scale flow reactor in co-HDT of mixed feed containing SRGO and 5, 10, and 15 wt% of SO. It was found that Co-PMo catalysts demonstrated higher hydrodesulfurization (HDS) activity in SRGO HDT than its Ni-PMo/C/Al2O3 counterparts. The use of C/Al2O3 led to more active Co-PMo and Ni-PMo catalysts compared with Al2O3 supported ones. Co-PMo samples more significantly lost activities than Ni-PMo after adding SO in the feed. Addition of SO led to an increase in HDS apparent activation energy by 21 kJ mol(-1) on Co-PMo and has not affected it in co-HDT of blended feedstock over Ni-PMo. All obtained results showed the high effectiveness of the prepared Ni-PMo/C/Al2O3 catalyst. They showed high HDS, hydrogenation and hydrodenitrogenation activities in co-HDT of the mixed SRGO/SO feedstock as well as catalyst stability against to accelerated deactivation. The possible reasons of the catalytic behavior of the studied samples are discussed based on the characteristics of the freshly sulfided and spent samples as well as determined kinetic parameters. (C) 2016 Elsevier B.V. All rights reserved.