This paper reports that the maximum electric field is a dominant factor for reliability in high-voltage GaN-HEMTs. Four types of the GaN-HEMT with different field plate (FP) structures were tested in continuous switching operation mode to analyze the degradation mechanism and the optimal device design. From the on-resistance degradation dependence on the FP structure, we extract that the gate-edge electric field strongly affects the increase of the dynamic on-resistance. Although the FP-edge field also increased the dynamic on-resistance, its influence was weaker than that of the gate-edge field. The optimal FP structure minimizes the increase of the dynamic on-resistance by reducing the electric field peaks and showed no degradation of power efficiency at the boost converter operation.