Abstract: To reveal the change of natural frequency of an iced transmission line during galloping,a nonlinear dynamic partial differential equation galloping in a plane for iced quad-bundled conductors was developed,and it was transformed into an ordinary differential equation using Galerkin method. The equation was solved by the average method,and the effects of balance point tension levels and galloping amplitude levels on nonlinear transient natural frequencies were analyzed. The results show that the transmission line balance tension level is enhanced because the galloping and the transmission line elastic coefficient changed with tension level lead to a change of the linear natural frequency; the galloping amplitude level has a great effect on higher-order nonlinear transient natural frequencies; the two roles result in the drifting of natural frequency during galloping,and the drift volume depends on wind speed and the span of a transmission line. The characteristics should be taken into consideration in the design of anti-galloping device.