Abstract: DWTT is the main item of fracture toughness for the pipe-line steel. The testing temperature has great effect on the DWTT results. The cooling medium choice is restricted due to there are not detailed regulations for temperature control as the specimen with gaseous cooling medium in the API, ASTM and the national standards. The analysis and calculating are made for the physical process of the cooling. The heat preservation time are provided for different temperature in the gaseous cooling medium. It provide primary sustaining base to control the cooling process of gaseous cooling medium. The essential point of the temperature testing is brought forward at the same time.