Many high performance aircraft suffer from a dynamic instability problem at relativelyhigh Mach number and altitude regimes known as Dutch roll instability. In this study theaircraft Dutch-roll and lateral –directional state variables may be enhanced using the yawdamper and washout filter. The Aircraft Dutch-roll damping is improved by identifying asuitable feedback gain matrix plus preserving the airframe's low-frequency behavior by theuse of a washout filter with a time constant. An intermediate change in terminology is used toillustrate the procedure for including a washout filter into the aircraft lateral state equationformulation. A stability augmentation system (SAS) and a numerical model constructed for aMig-21 aircraft are solved to illustrate the proposed procedure. The numerical results showthat it was sufficient to use (SAS) including both yaw damper and washout filter to improvethe aircraft sideslip angle, roll rate and roll angle response in a short time. Finally, theverification of the aerodynamic characteristics of present model was made by comparing theresults of the method used (Low order three dimensional Panel method) with that of Cy-20aircraft flight data test.