Based on the general approach of Incremental Harmonic Balance method (IHBM), the behavior of nonlinear dynamic stability for viscoelastic plates subjected to inplane periodic excitation was investigated by converting the integrodifferential governing equations to differential ones. The dependence of the downward shift and contraction of principal instable regions on the viscose parameters of standard linear solid and vibration frequency of plates was examined. A simplified approach of direct application of Incremental Harmonic Balance method to integrodifferential equation of motion was given, and this approximate method was validated through the comparison of results obtained respectively by two kinds of application of IHBM.