Daily gas exchange, stomatal conductance and water use efficiency in summer leaves of 6-8 years old Ormosia pinnata were undertaken in a mixed forest of Heshan Hilly Land Interdisplinary Experiment Station in 1995. Results showed that the daily variations of net photosynthetic rate (Pn) and transpiration rate (Tr) exhibited a two-peak pattern. Pn was mainly affected by light, and had no obvious correlation with air temperature, leaf temperature and humidity. Tr correlated with light, air temperature and leaf temperature positively，but negatively with the humidity. Stomatal conductance appeared more seusitive to humidity than to light and temperature．Stomatal conductance and Pn responded independently to environmental factors．Water use efficiency of Ormosia pinnata peaked in the early morning and demonstrated a significant correlation with stomata1 conductance in the daytime．