Abstract: In this paper, using the minimum singular value of the system return difference matrix and universal gain phase margin diagram, the relations between stability margins and the system quality of multivariable flight control systems have been studied. The results indicate that the stability margins are different when L is introduced at the plant input and output. When L is introduced at the plant input, the result of system robustness is equal to that of structure singular value analysis and when L is introduced at the plant output, the result is equal to that of complement sensitivity singular value analysis. The stability margins evaluated by introducing L at the plant output probably reflect the system quality and the relation between them is the same as that of SISO system.