Abstract: The diurnal variation of photosynthesis for leaves of mulberry in desert had been studied under the natural conditions. The results were as follow: when it is strong photic, hyperthermal and droughty, the curves of diurnal variations of net photo-synthetic rate（Po） had two peaks. Po and stomatal conductance（Cs）are a little low. Po had a significant correlation with Cs. It was known that the photosynthesis in leaves was influenced by activity of stomatal factors, transpiration rates, external air temperature and relative humidity and so on.