Gallium Nitride (GaN) devices are becoming an attractive option in industry to achieve high-power-density and high-efficiency converter design. However, the dynamic on-resistance of GaN devices still remains a major concern. In this paper, a new dynamic on-resistance measurement circuit is proposed for fast-switching GaN power HEMTs. Simple circuit design, fast sensing speed, and accurate measurement can be realized with exclusively passive components. Compared with existing circuits, detailed component selection guidelines to realize fast sensing speed is discussed considering the switching transients. Simulation results show an accurate tracking of on-resistance in 48.3 ns after the drain current reaches the load current in the turn-on process. The proposed circuit is implemented in a double-pulse test for a 650 V GaN device. Experiment results demonstrate that the proposed circuit can work effectively at various gate resistance, dc-link voltage and load current. At 300 V/15 A turn-on process with R _g,on = 20 Ω, the circuit can accurately obtain the dynamic on-state voltage within 49.6 ns after the load current is reached. Under the same switching condition with R _g,on = 0 Ω, a faster sensing speed is achieved, and the circuit is able to track the dynamic on-resistance in 47.6 ns after the drain current starts to rise.