In this paper, a field-plated III-nitride Nano-HEMT over the β-Ga2O3 substrate is designed with three different types of gate structures. The impact of different gate structures on transfer characteristics, drain characteristics, electric field distribution, and gate leakage characteristics of the proposed nano-HEMT are presented with profound analysis. The device with a double-head (DH) recessed gate length of 20nm exhibits better transfer characteristics, pinch-off characteristics, and lower gate leakage current. It is believed that this piece of research will give an insightful thought on the improved III-nitride HEMT grown on the emerging β-Ga2O3 material as a substrate-based transistor technology.