Gallium nitride enhancement-mode high electron mobility transistors (GaN E-HEMTs) can achieve high frequency and high efficiency due to its excellent switching performance compared with conventional Si transistors. Nevertheless, GaN HEMTs exhibit a more pronounced dynamic ON-state resistance R-{mathrm {DS}(mathrm{scriptscriptstyle ON})} than silicon transistors. The variation of R-{mathrm {DS}(mathrm{scriptscriptstyle ON})} is caused by both the static R-{mathrm {DS}(mathrm{scriptscriptstyle ON})} due to junction temperature rise and the dynamic R-{mathrm {DS}(mathrm{scriptscriptstyle ON})} due to the electron trapping. Without a careful decoupling analysis, it is difficult to calculate and model the dynamic R-{mathrm {DS}(mathrm{scriptscriptstyle ON})} portion. This article introduces a comprehensive approach of dynamic R-{mathrm {DS}(mathrm{scriptscriptstyle ON})} evaluation comprising four techniques: 1) a clamping circuit for both the hard-switching (HS) device and synchronous rectification (SR) device; 2) a junction temperature monitoring technique; 3) control of both the pulse test and soak time; and 4) continuous operation of device under test. Based on the dynamic R-{mathrm {DS}(mathrm{scriptscriptstyle ON})} test results, a new model of the R-{mathrm {DS}(mathrm{scriptscriptstyle ON})} variation is developed where two coefficients k-{T{j}} and k-{mathrm {dR}} are defined to model the contribution of the heating effect and the impact of the trapping effect, respectively. The R-{mathrm {DS}(mathrm{scriptscriptstyle ON})} model is validated by the comparison between the calculated and measured junction temperatures of a 650-V/30-A GaN-based half-bridge. Furthermore, a detailed loss breakdown analysis is conducted for the GaN-based HS half-bridge. The results show that the switching losses, E-{mathrm{scriptscriptstyle ON}} and E-{mathrm{scriptscriptstyle OFF}} , are the dominant loss factors with high switching frequency. At last, the possible efficiency improvements are also discussed in detail.