The process of co-hydrotreatment of straight-run diesel fraction (DF) and vegetable oil (VO) on Co(Ni)-PMo/Al2O3 catalysts prepared from H3PMo12O40 and cobalt (nickel) citrate has been studied. It has been shown that under conditions close to those in the industry, the complete conversion of fatty acid triglycerides (FATG) is achieved on the catalysts of both types to give an ultraclean hydrotreating product in a 97% yield and a cetane number of 5 points above that of the hydrotreating product of the DF alone. The degree of hydrodesulfurization (HDS) is reduced more significantly on the Co-PMo/Al2O3 catalyst than in the case of Ni-PMo/Al2O3. The catalysts are more susceptible to deactivation in the hydrotreating of the blended feedstock containing VO. The Co-PMo/Al2O3 sample is less stable than Ni-PMo/Al2O3. Examination of the spent catalysts by transmission electron microscopy has shown that the average particle length of the active phase of Co-PMo/Al2O3 increases, whereas this increment for Ni-PMo/Al2O3 is insignificant, indicating higher stability of particles of the NiMoS phase. Thus, the co-hydrotreating of petroleum fractions and vegetable oil is more reasonable to carry out on NiMo/Al2O3 catalysts.