Brine heater is one of the main components in seawater desalination process used to heat seawater using saturated steam from HP steam header to a certain temperature (top brine temperature), in which the output water temperature from the brine heater may affect the evaporation and condensation processes of the evaporator. Brine heater is a heat exchanger type shell and tube with a parallel flow type whose the effectiveness can affect the performance of desalination plant. This study aims to show the effect of brine heater effectiveness on the performance of desalination plant. The Research Methods that used are observation, data research and calculation of brine heater effectiveness, interview and discussion with the related parties and then analyze the possible effect of brine heater effectiveness on the performance of desalination plant based on data. The results showed that the effect of brine heater effectiveness on desalination plant performance is enough significant, in value of 53%, but there were another factors that affected Desalination Plant performance, the factors were the performance of evaporator and ejector. The value effectiveness and performance of Desalination Plant unit 2 show a decrease caused by the crust buildup and the difference load of the PLTGU unit.