This paper discussed the effects of different water potential on the equilibrium time of leaf chamber and cooling time of thermocouple. The results measured by dew point and psychrometric methods, by in vitro and in situ, and by two kinds of measurement techniques of osmotic potential were compared. Also the in vitro changes of Yw，Ys and Yp in leaves of barley under NaCI stress was measured, and the in situ day and night changes of Yw，Ys and Yp in the leaves of wheat and bean under field condition were estimated.