We have observed dynamic scaling behaviour of hysteresis loop areas in KDP crystals as a function of applied field amplitude $E_0$ and frequency Ω. Our results exhibit a uniform power law scaling behavior $A~E_0^{1.55}ㆍΩ^{0.20}$ in the region of low $E_0$ and Ω, compatible with the Steinmetz law and the result of Hashimoto et al. This result shows the possibility that both ferroelectric and ferromagnetic bulk systems may have the same universal behavior and their real structure such as domains, anisotropy, defects it etc. which all theories have not considered may have significant effects on dynamics of their hysteresis loop switching. And it clearly shows a distinctive difference between thin films and bulk systems.