Under natural dry conditions, diurnal changes of photosynthetic characteristics of Stipa bungeana and Panicum virgatum were measured with the Li-6400 portable photosynthesis system. The results showed that the Panicum virgatum had bigger photosynthetic rate, water use efficiency, smaller transpiration rate and strong stomata adjustment ability. Stipa bungeana had smaller photosynthetic rate and water use efficiency. Compared with Stipa bungeana，Panicum virgatum's higher photosynthetic rate may be caused by its high Rubisco activity because of its relative smaller stomata conductance.