The membership functions are the critical factors of the system performance for fuzzy logic control. The degree of membership is determined by a membership function, which is defined based on experience or intuition of engineers. In general, it is accepted that membership functions changes several times as the system is tuned to achieve desired responses to given inputs. Once the system is in operation, the membership functions do not change. So the membership functions must be tuned properly to obtain the desired system performance. In this paper, we propose the technique to overcome the tuning difficulties of membership functions by trial and error method. This technique has been applied to the system with parameter uncertainties to investigate the performance.