Fuzzy logic control law to improve duch roll characteristics of aircraft was designed and its control performance was evaluated. First, control law was designed for a small high speed aircraft at low altitude and low speed flight condition. Then the control law was applied to flight conditions from the minimum speed to the supersonic and sea level to high altitude, and these control performances were evaluated. Additionally, this control law was adapted to a large transport aircraft without any parameter change. The evaluation showed good control performance to improve the duch roll characteristics of all flight conditions of both small high speed aircraft and large transport aircraft without the parameter change. This meant that the fuzzy control proved to provide effective flexible application to aircraft stability augmentation. In case of actual flight in strong air turburence, input to fuzzy logic controller may exceed the limit of effective range. To cope with this problem, the countermeasure method was provided and proved its effectiveness.