Abstract: When sine voltage source u is applied to the proper port of networks, the fundamental harmonic component iS1 of injecting current of networks can be found by fundamental-wave balance principle, The active component of input current is represented by the input conductance Gi. It can be proved that the stability of networks depends on the sgnGi. The Fundamental-wave balance principle can be spread and applied to the third-order nonlinear network. The universality of the conclusion can be spread to mathematics and mechanics science also and give a popular method to solving this third-order nonlinear differential equation qualitatively.