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Gallium Nitride (GaN) transistors are emerging as promising candidates for making high-frequency, low-loss and small-size power converters. To realize normally-off, p-GaN gate technique is widely adopted in commercially available GaN-based power devices. However, owing to the distinctions in device structure, the intrinsic capacitances with regard to gate region vary from those of Si MOSFET. Besides, with drain-bias rising, the variation of gate region’s net charge could make the threshold voltage of GaN transistor unstable. Thus, the switching transient waveforms of GaN transistor could be significantly influenced by the aforementioned factors, and the commonly used analysis method for Si MOSFET would not be sufficient. In this work, the threshold voltage instability is firstly analyzed, which is related to drain-to-gate voltage stress. Due to the difficulties in directly measuring the gate-related capacitances and their dynamic behaviors, a hybrid physical-behavioral modeling method is proposed, which is capable of extracting the relationship between the gate-related capacitances and their bias from the static measurements. The proposed analysis methods are then implemented on a GaN-based phase-leg circuit. Through the comparison with the experimental results and the simulated waveforms of the most advanced analysis, the proposed analysis approach exhibits outstanding performance. © 2017 IEEE.