Abstract: A comparative research on the sealed-bottle weighing method, leaf quick-weighing method and porometer method was conducted in a poplar plantation (Populus deltoides Bartr. cv. "Lux" ex. I-69/55) in 1983~1986. The results have revealed that the deviation of transpiration value determined by quick-weighing method was relevant to the meteorological factors and the water status of leaves. The process of lossing water from leaves can be divided into four stages. A discussion for determining transpiration rate at each stage has been made.