This paper presents a method for finding the boundaries of gain margins and phase margins of multivariable control systems with adjustable parameters. The considered systems are first decomposed into the augmented open-loop transfer functions modified by adding an analytical gain-phase margin tester, then the characteristic equations are formulated, and finally the stability equations are used to find the boundaries of constant gain margins and phase margins. The main advantage of the proposed method is to obtain complete information about the effects of adjustable parameters on gain margins and phase margins; thus it is a useful tool for analysis and design of robust control systems,