Abstract: Diurnal variation of the photosynthesis, transpiration,water potential and conductance to water vapour in the soybean leaf have been examined simultaneously with photosynthetically active radiation, leaf temperature, leaf to air water vapour concentration difference and CO2 concentration of air surrounding the sample leaf by Minicuvette System under field conditions and controlled conditions for two years.The results indicate that the diurna 1 changes of measured physiological parameters showed a one-or two-peak-changing pattern. It was further clarified that such changing patterns took place not because of genetic physiological rhythm, but resulted from the effects of diurnal changes of ecological factors on the physiological characters. In the present experiments, the main ecological factors were photosynthetically active radiation leaf temperature and leaf to air water vapour concentration difference.