In recent years, GaN power HEMTs have been adopted in emerging power applications (e.g. fast charger, LiDAR & Datacenter etc.), where its unique superiority of high switching speed endows the GaN-based power system with significantly increased power density. However, the increased power density may lead to severe self-heating effect that could result in device/system performance decline or even cause device reliability degradation. In this work, from both device and system perspectives, based on the typical 200 V-48 V half-bridge buck converter platform, the self-heating dynamics of the GaN power HEMTs and its impact on the power performance of the system is studied. Prominent self-heating effect of GaN HEMTs up to ~150 °C during circuit operation is observed, which leads to much higher power loss compared with the well-known current collapse (i.e. dynamic Rds,on). At circuit operation frequency of 200 and 500 kHz, the additional power loss caused by self-heating effect is 0.89 and 1.68 W, respectively, accounting for 42.4% and 58% of the total loss of the circuit. The results indicate that delicate thermal management is critical for GaN power system to achieve high power efficiency while pursuing higher power density.