Hysteresis loop area A was observed in ferroelectric KDP crystal as a function of the field amplitude E and frequency Omega to derive a scaling behaviour of A proportional to E(1.55) Omega(0.20) (alpha=1.55, beta=0.20) in small field and low frequency limits. The old Steinmetz law, which has been in disparity with both recent theoretical values and related thin film experiments, is found to be obeyed in our ferroelectric hysteresis curves of a KDP bulk crystal when we prevent our measurements properly from the distortion due to the resistive dielectric loss by going to low temperature below the domain wall freezing temperature.