Method for treating petroleum heavy oil

A method for producing cracked oil and homogeneous coke of higher fixed carbon with a higher yield by treating petroleum heavy oil selected from the group consisting of atmospheric residue, a vacuum residue, thermally-cracked residue, catalytically cracked residue, solvent deasphalted residue, tar sand bitumen and crude shale oil, which comprises subjecting said petroleum heavy oil to heat treatment at a temperature of 400.degree.-550.degree. C. by continuously feeding an additive selected from the group consisting of disulfides, benzothiazoles or metal salts thereof, metal salts of alkylxanthogenic acids, metal salts of arylxanthogenic acids, metal salts excluding antimony of dialkyldithiocarbamic acids, metal salts excluding antimony of diaryldithiocarbamic acids, metal salts excluding antimony of dialkyldithiophosphoric acids and metal salts excluding antimony of diaryldithiophosphoric acid to said petroleum heavy oil.