Heat recovery steam generator or HRSG is one of the big equipment that is productively being used these days to recover the heat and to produce it into steam which can be used in many applications. However, it is known that HRSG is a complex system with highly nonlinear characteristics and using conventional controller to control the system has brought a lot of drawbacks. Hence, to overcome these drawbacks, Fuzzy Logic Controller is employed to control the HRSG complex system. This paper discusses the design of a Fuzzy Logic Controller with three different membership functions which are 5, 7, and 9 that were used as pressure controller to control the input to HRSG based on the steam demand. The performances of the fuzzy logic controller were then compared with the classical PI controller. © 2011 IEEE.