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Heat gain through the windows is of great importance as it is the main part of cooling load in the hot summer and cold winter climate. This paper attempts to find out the most efficient energy saving technology for windows by analysing the thermal process of the south and west oriented windows in Shanghai. By numerical calculation, the ratios of different compositions are compared to find out which is the main source of heat gain of windows. The calculation result shows that solar radiation heat gain is much bigger than the heat transmission caused by temperature difference between indoor and outdoor air. Solar heat gain for west windows is much higher than for south windows after 12:00. For south windows, diffuse radiation and reflected radiation from ground are the main source of solar heat gain. As for west windows, direct radiation heat gain takes up most of solar heat gain after 12:00. Therefore different strategies should be taken according to different orientations. [ABSTRACT FROM AUTHOR]