Global warming means not only increase in average air temperature but also increase in frequency of extreme weathers such as extremely high temperature over 45oC. Crops are generally sensitive to high temperature during their reproductive growth stage. This experiment was thus conducted to investgate physiological responses of rice (Oryza sativa cv. Chucheong) and soybean (Glycine max cv. Sinpaldal) to high temperature (HT) stress at their reproductive stage. Rice and soybean were exposed to different degrees of high temperature stress by keeping them in a growth cabinet deisnged to maintain air temperature up to 45oC for at least 5 hours in a day for different durations, 0, 1, 2, 3, and 4 days. HT stress treatment delayed heading and flowering of rice and increased the sterility of its' main panicle with increasing duration of HT treatment; 19.7 43.4, 68.1, 81.5 and 91.1% at 0, 1, 2, 3 and 4 days HT treatment, respectively. Increasing sterility due to HT treatment thus resulted in significant rice yield loss; 5.07 4.27, 4.32, 4.51 and 3.62 g/plant at 0, 1, 2, 3 and 4 days of HT treatment, respectively. Soybean was also significantly affected by HT stress in its pod formation and sterility, particularly along the vertical stem. Increasing pod sterility with increasing HT treatemnt thus resulted in significant soybean yield loss.