FAO-56 Penman-Monteth (FPM) model is one of the most applied methods to estimate crop evapotranspiration; yet the accuracy of the model in greenhouses is still undetermined especially in non-standard conditions of water deficit and salinity. This study focused on the performance of the FPM model in estimating greenhouse and outdoor eggplant evapotranspiration (ET_c), crop coefficient (Kc) and water stress coefficient (Ks) under different levels of water deficit and salinity. An area in the greenhouse and the outdoor field was assigned to grass cultivation as a reference crop. Daily reference (ET₀) and crop evapotranspirations were measured by diurnal weighting of microlysimeters throughout the growing season (from May 19th to September 5th, 2012). The performance of the FPM model was evaluated by four statistical difference criterions along with regression indices. The ET₀ values were properly estimated in the outdoor conditions but showed about 12 percent underestimation in the greenhouse; however, the trends of daily ET₀changes were well predicted in both environments. Apparent differences in daily ET_c variations were met during the growing season, for various levels and combinations of water stress. Best estimations of daily and mean 10-day ET_c values were those of daily irrigated (I₁) treatments in both environments. In mean 10-day ET_c, better correlations were obtained between the measured and estimated values due to smoothened fluctuations in weather and data and soil moisture changes. Actual and calculated values of Ks along with the correctness of their estimations decreased with the intensity of water stresses in both environments. The variation patterns of daily Kc values was similar to those of the daily ET_c. Mean 10-day values of Kc were properly estimated by the FPM model; yet outdoor estimations were accurate in all treatments.