Heat exchanger is an important component in the Gas Turbine Power Plant (GTPP) industry, such as Turbine Cooling Air (TCA) Cooler and Fuel Gas Heater. They are heat exchanger with cross flow type arranged into one part and used the ambient air as a cooling medium. The principle that the fan will draft the ambient air to the TCA Cooler to cool the hot air with a temperature of 400 - 467.3 oC from the last stage compressor discharge is used as a coolant on the turbine blades. The ambient air which came out from the TCA Cooler was have a temperature range 200 - 250 oC , then it would be utilized by the fuel gas heater to heat the fuel gas before entered to the combustion chamber, then, combustion process is worked perfectly and it would be increased the thermal efficiency in the gas turbine. Changes in ambient temperature greatly affected to the performance of the heat exchanger. Therefore it is necessary to calculate the effectiveness and rate of heat transfer with the method of Thermal of Unit - effectiveness and Log Mean Differential Temperature (LMTD) method.