A line of tube transfering 300°C vapour at high pressure has been surveyed by means of thermography to obtain its surface temperature. The tube is 529 mm in diameter. A great number of thermal images are observed and analysed. Heat leakage is shown by much higher temperature area on these thermal images. One can find different causes of these heat leakage, for example, from imperfect parts of insulation meterial around tube, or from the connection gap between block-shape insulation material. A thermal imaging system with capability of digital processing image is used. A practical method to calculate the heat flux density from the image is developed. The program is executed in the same time with the measurement. Such quantitative results may be compared in the field with other ordinary measurements, such as heat flux meter, and enthalpy variation calculation. Over 1.62km distance, a variety of thermal patterns are processed digitally, an average of heat flux density 110 kcal/m2h is obtained, which is in good coincidence with other measurements. Other results, such as the highest and average surface temperaturea,the percentage of heat leakage contribution from different areas, etc., are also obtained. This digital processing method can provide a standard to check the quality of heat insulation. Experiments with different insulation materials are also made, and useful results are obtained from their comparison.