Abstract: A catalyst composition for the hydrogenation of heavy hydrocarbon oil comprising at least one active ingredient for hydrogenation supported on a porous refractory oxide carrier and which has the following characteristics:(1) the mean pore diameter of the pores thereof is in the range of 130 to 250 .ANG.;(2) the volume of pores having a pore diameter within .+-.30 .ANG. of the mean pore diameter is in the range of from not less than 30% to less than 60% of the total volume of the pores;(3) the volume of pores having a pore diameter of not larger than 80 .ANG. is not more than 13% of the total volume of the pores;(4) the volume of pores having a pore diameter of not smaller than 350 .ANG. is not more than 25% of the total volume of the pores; and(5) the ratio (mm.sup.2 /mm.sup.3) of the outer surface of a molded catalyst particle to the volume thereof is in the range of 3 to 8.