Abstract: The daily gas exchange,stomatal conductance and water use efficiency of Platycladus orientalis in the test area of Fangshan Country of Shanxi Province in a semi-arid region of the Loess Plateau were measured with a portable Li --6200 gas analysis system under natural conditions.The results show that the diurnal course of net photosynthetic rate displays a two-peak pattern,that of stomatal conductance a hollow pattern and that of transpiration rate a signal-peak pattern.Water use efficiency culminates in the early morning.On the basis of two criteria of stomatal limitation of photosynthesis suggested by Farquhar and Sharkey,the predominant limiting factor of photosynthesis is stomatal conductance of stomatal limitation in the morning (10:00? ?2:00).The midday depression of photosynthesis at noon(12:00—14:00) and the decrease of photosynthesis in the afternoon (16:00—18:00)are the result of the low carboxylation capacity of the leafmesophyll.