Abstract: Stability of a fuzzy PID controller with one input, which is proposed recently, is studied in this paper. Fuzzy logic inference in the fuzzy PID controller is nonlinear, therefore,-Passivity theorem is utilized to analyze its stability. The relations between parameters are given to guarantee stability of the whole system with respect to the first order plant and the second order plant. A simulation example is shown to verify the results. Stability of the higher-order plants can also be analyzed by-passivity theorem.