Abstract: In this study, a simple theoretical framework was extended to account for the effects of temperature and atmospheric CO2 concentration on RUE. A general test of the model showed that simulated and observed daily biomass productions under average daily temperatures ranged from 11 to 28°C are similar. The calculated RUEs for 13-23°C were similar to measured ones and percentage increase in RUE for CO2 concentration of 640 ppm relative to 330 ppm was in agreement with the measured percentage increase in biomass production. By using the framework, RUE response functions to average daily temperature and CO2 concentration were calculated for chickpea. RUE of chickpea at temperatures lower than 3°C and higher than 36°C is zero. RUE rapidly increased (9.5%°C-1) with increasing temperature from 3 to 14°C. With further increase in temperature to 22°C, RUE slowly (1.4%°C-1) decreased and temperature increase between 22 to 36°C resulted in sharp decrease (7.4%°C-1) in RUE. Response of RUE to CO2 concentration was curvilinear. At low concentrations of CO2 (60 to 400 ppm), RUE was especially sensitive to increases in CO2 concentration, but increases in CO2 greater than 700 ppm were predicted to result in only small increases in RUE. The functions obtained can be used in simulation studies of chickpea crop response to projected climate change.