The influence of mesa edge capacitance on the frequency characteristics of AlGaN/GaN HEMTs with 90nm gate length was studied in this paper. To extract mesa edge capacitances, a small-signal equivalent circuit model considering mesa edge capacitances was provided. Based on the model, the intrinsic gate capacitances of AlGaN/GaN HEMTs with 2×20μm, 2×30μm, 2×40μm, and 2×50μm gate widths were extracted, respectively. Through linear fitting along gate width for the extracted results and simulations, 8.06fF/μm2 of mesa edge capacitances at Vgs=−4.5V and Vds=8V in the devices with 2×20μm gate width was obtained, which can be about 33.2% of the total gate capacitance. Mesa edge capacitances results in a significant drop of current-gain cut-off frequency (fT), and the effect is more serious in the shorter gate length devices.