Using speed and power output deviations as the controller input variables, a fuzzy logic based controller (FLC) has been developed to perform the function of a power system stabiliser and to provide a supplementary signal to the excitation system of a synchronous machine. The complete range for the variation of each of the two controller inputs is represented by a 7*7 decision table, i.e. 49 rules. The FLC design steps and a procedure for tuning its parameters are described. Simulation studies for a variety of disturbances on the power system with the FLC-based power system stabiliser demonstrate its effectiveness in improving system performance.