Technical committee for Japanese industrial standard of heat treating oils has studied on the new method for testing cooling power of aqueous quenchants. This method employed the cooling curve test of a new sliver rod probe, which has a metal-sheathed thermocouple at its geometrical center. The experimental study has clarified that this probe has the higher sensitivity and excellent durability. In order to standardize this probe for cooling power test of aqueous quenchants, the repeatability and reproducibility of test results etc. have been studied. D.O.P. (Di-2-ethylthexyl Phthalate) was used as reference quenching fluid for initial calibration and for periodic calibration verification of the probe and system. In addition, 10% brine was used to check the response capability of the probe and system to the sudden change of temperature. These procedures are established according to the experimental investigation. This new test method is going to be built in the revised version of JIS K 2242 “Heat treating oils” as the cooling power test method for aqueous quenchants. Technical committee for Japanese industrial standard of heat treating oils has studied on the new method for testing cooling power of aqueous quenchants. This method employed the cooling curve test of a new sliver rod probe, which has a metal-sheathed thermocouple at its geometrical center. The experimental study has clarified that this probe has greater sensitivity and excellent durability. In order to standardize this probe for cooling power test of aqueous quenchants, the repeatability and reproducibility of test results etc. have been studied. DOP (Di-2-ethylthexyl In addition, 10% brine was used to check the response capability of the probe and system to sudden change of temperature. These procedures are established according to the experimental investigation. This new test method is going to be built in the revised version of JIS K 2242 “Heat conditioned oils ” as the cooling power test method for aqueous quenchants.