Steam Turbine is one of the main equipment that takes an important role in the process of Gas and Steam Power Plant (PLTGU). The function of Steam Turbine is to expand superheat vapor to produce mechanical energy to drive the generator which will generate electrical energy. In Steam Turbine, there is a Heat Exchanger which aims to cool the oil temperature to be used as lubrication and cooling bearing on the turbine shaft. The temperature of the oil is kept so that the bearings are not overheated and wear out. There is a difference in the use of Heat Exchanger type on Steam Turbine. In the Gas and Steam Power Plant (PLTGU) X, the type of Heat Exchanger is Plate type. While in the Gas and Steam Power Plant (PLTGU) Y, the type of Heat Exchanger is Shell and Tube type. This analysis aims to determine the most suitable and most effective Heat Exchanger to be used on Steam Turbine. To know the ratio of heat transfer and effectiveness of Heat Exchanger of each type, calculation method of NTU and LMTD with data from each type of Heat Exchanger were used. In the Gas and Steam Power Plant (PLTGU) X, the oil drop temperature after passing the Heat Exchanger with Plate type has a value ranging from 10o. While in the Gas and Steam Power Plant (PLTGU) Y, the temperature of oil decrease after passing Heat Exchangers with Shell and Tube type has a value ranging from 8o. After the analysis, Heat Exchanger with Plate type is more suitable and effective for use on Steam Turbine compared to Heat Exchangers with Shell and Tube type due to be able to lower the oil temperature up to 10o. Heat Exchanger with Plate type is also more flexible because it can be easily done for addition to its Plate.