A phenomenological model was proposed which describes frequency dispersion of nonlinear dielectric response of relaxer ferroelectrics (relaxors) as a result of dispersion of their linens dielectric permittivity. The model was applied to Pb(Mg1/3Nb2/3)O-3 (PMN) relaxer. It provided a good qualitative description of temperature and frequency dependence of the third harmonics of PMN. Analysis within the model yielded a frequency independent nonlinear coefficient corresponding: to static nonlinear dielectric susceptibility. The model explained the recently reported for PMN data on the Vogel-Fulcher law for frequency dependence of the temperature at which the third harmonics passes a maximum.