Abstract: The transpiration rate,stomatal resistance and environmental factors of Platycladus orientalis in the test field of Fangshan Country of Shanxi Province in the semi-arid region of the Loess Plateau were measured with a portable steady porometer(Li-1600).The study provides theoretical basis in analyzing the transpiration response on the environmental factors and the optimum water ecological condition of Platycladus orientalis in semi-arid region.The results show that the diurnal course of transpiration rate displays a double-peak pattern.The transpiration rate presents a cubic relation with water content in soil and an exponential relation with stomatal resistance.The transpiration rate correlates significantly with the main environmental factors such as air temperature,relative humidity and photosynthetic active radiation.The air temperature is the main factor which affects transpiration.