Abstract: The concepts of equilibrium solution, initial variation and feedback gain of circuit variables are introduced. A stable or unstable circuit can be explained with the tendency of initial variation of circuit variables relative to the equilibrium solution, and can be discriminated on the feedback gain. The fundamental equations and some coeffi cient determinant theorems are derived, one equation being used for solving the feedback gain,two discrimination theorems being used for determining the value bound of the feedback gain. Further, the stability of non ideal models of resistance circuits and circuits with capacitors and inductors are discussed. In order to compile the circuit equations, independent loop method and independent closed surface method on circuits are presented.