The development of infrared thermometry has led many researchers to use plant temperatures, and specifically the temperature of the crop canopy in the field, for estimating the water stress of a crop. The purpose of this study was to evaluate the role of leaf temperature in irrigation scheduling. An experiment was carried out in a greenhouse with chinese cabbage. Leaf temperature was measured with infrared thermometry and evapotranspiration of the crop was measured by lysimeters. Influence of the difference between leaf temperature and air temperature on crop evapotranspiration was evaluated under varying water stress condition. A further objective was to evaluate the effect of other climatic variables on the relationship between evapotranspiration and temperature difference between leaf and air. A statistical model for estimating evapotranspiration using the temperature difference, relative humidity. and radiation was developed and tested. Crop water stress index was calculated using vapour pressure deficit and the temperature difference. Relations between the crop water stress index and crop evapotranspiration was tested. The index was closely related with evapotranspiration.