The major task in compact modeling for high power devices is to predict the switching waveform accurately because it determines the energy loss of circuits. Device capacitance mainly determines the switching characteristics, which makes accurate capacitance modeling inevitable. This paper presents a newly developed compact model HiSIM-GaN [Hiroshima University STARC IGFET Model for Gallium-Nitride-based High Electron Mobility Transistors (GaN-HEMTs)], where the focus is given on the accurate modeling of the field-plate (FP), which is introduced to delocalize the electric-field peak that occurs at the electrode edge. We demonstrate that the proposed model reproduces capacitance measurements of a GaN-HEMT accurately without fitting parameters. Furthermore, the influence of the field plate on the studied circuit performance is analyzed.