A heat treatment oil composition comprising a mixed base oil containing 50-95 weight % of (A) a low viscosity base oil with kinematic viscosity of 5-60 mm.sup.2/s at 40.degree. C., and 50-5 weight % of (B) a high viscosity base oil with kinematic viscosity of more than 300 mm.sup.2/s at 40.degree. C. is proposed. By the use of this heat treatment oil composition for hardening of metallic material, it enables to generate little cooling unevenness, to assure the hardness of hardening processed product, and to reduce quenching distortion.