Efficiency and electric-power output of gas turbines vary according to the ambient conditions. The amount of these variations greatly affects electricity production, fuel consumption and plant incomes. Since ambient conditions are dependent upon the place where gas turbine is installed, they cannot be changed. At the same time, the amount of performance variation with the ambient conditions also depends on the gas turbines design parameters. Therefore, in order to determine the actual performance variation with the ambient conditions, gas turbine design parameters and ambient conditions of the installed place should be known. For this purpose, two gas turbine models and seven climate regions of Turkey are considered in this study. For both two models, by using average monthly temperature data of the regions, annual electricity production loss and fuel consumption increase compared to those in standard design conditions (sea level, 15 degrees C, 60% relative humidity). Electricity production loss is about 2.87-0.71% take place, compared to the standard annual production rate in hot regions. Electricity production loss occurs in all regions during the periods when the temperature is above the 15 degrees C standard ambient temperature and loss rates vary between 1.67% and 7.22% depending upon the regions. Electricity generation increases for about 0.27-10.28% when inlet air is cooled to 10 degrees C. (c) 2005 Elsevier Ltd. All rights reserved.