We have systematically investigated the dynamic susceptibility spectra of square-patterned ferromagnets in the mesoscopic scale from 100 nm to 500 nm by means of a micromagnetic simulation. The dynamic susceptibility spectra are obtained from the magnetization response under the applied field using Fourier transforms technique. We have observed that the frequency peak of the susceptibility spectra slightly increases as the length of the square-patterned increases. Interestingly, the frequency peak shows similar behavior compared to the frequency resonance from Kittel's formula. We have also analyzed the magnetic energies, where the demagnetization energy is larger than the exchange energy. It can be explained that the polar interaction primarily contributes to the resonance mode of the square-patterned ferromagnets.