A scheme for on-line tuning of a fuzzy-logic power system stabilizer (FPSS) is presented. Firstly, a FPSS is developed using speed deviation and accelerating power as the controller input variables. The inference mechanism of the fuzzy-logic controller is represented by a decision table, constructed of linguistic IF-THEN rules. The linguistic rules are available from experts and the design procedure is based on these rules. It is assumed that an exact model of the plant is not available and it is difficult to extract the exact parameters of the power plant. Thus, the design procedure can not be based on an exact model. This is an advantage of fuzzy logic that makes the design of a controller possible without knowing the exact model of the plant. Secondly, two scaling parameters are introduced to tune the FPSS. These scaling parameters are the outputs of another fuzzy-logic system, which gets the operating conditions of the power system as inputs. This mechanism of tuning the FPSS makes the FPSS adaptive to changes in the operating conditions. Therefore, the degradation of the system response, under a wide range of operating conditions, is less compared to the system response with a fixed-parameter FPSS and a conventional (linear) power system stabilizer (CPSS). The tuned stabilizer has been tested by performing nonlinear simulations using a synchronous machine-infinite bus model. The responses are compared with a fixed-parameters FPSS and a CPSS. It is shown that the tuned FPSS is superior to both of them.