In this work, we proposed a step graded junction termination extension in vertical GaN pn diodes using hygroden treatment. The carrier modulation effect by H treatment and following thermal diffusion is successfully observed by conductance and KPFM measurement, from which the diffusion coefficient of H in p-GaN is also extracted. Finally, a 1500 V vertical GaN pn diode is achieved with the proposed edge termination. The paper can be accessed by: https://doi.org/10.1109/LED.2021.3091335.