A new method for estimating crop transpiration and crop water stress index (CWSI) by using a temperature-related approach was developed and verified this study. In the proposed method, 3 temperatures and a few meteorological variables (net radiation and humidity) are required. The 3 temperatures include sunlit leaf temperature, imitation leaf temperature, and air temperature. The imitation leaf refers to an artificial leaf without transpiration. The major advantage of the proposed method is that plant correction factors, empirical coefficient, and canopy and aerodynamic resistances are not required. Experiments for verification were conducted in a sandy field with sorghum crop and in a greenhouse with melon crop. Data obtained by the proposed method were compared with the amount of transpiration determined with lysimeters, soil water status determined by TDR, and CWSI estimated by Jackson’s method. The calculated transpiration by the proposed method was consistent with the results obtained with lysimeters, with regression coefficient 1²=0.88. Values of CWSI calculated by the proposed method were in agreement with Jackson’s CWSI, with a regression coefficient approximately equal to 1. These results indicate that the proposed method can be applied to estimate crop transpiration and water stress index under both field and greenhouse conditions.