Abstract: In the light of physics and basic equations of GaN-based high-electron mobility transistors (HEMT) with a field-plate, an analytic model of the surface electric field distribution and peak electric field are achieved. According to our model, when the field-plate length LFP and dielectric layer thickness tox are optimized, the peak electric field decreases to 22% compared with the condition with no field plate. With a well designed field plate applied, the maximum drain voltage can be improved from 50 V to 225 V while the peak electric field remaining unchanged. The results obtained from the model agree with those from the numerical simulation reported recently. The conclusions afford a good theoretically guidance to the design for GaN HEMT with field plate.