We report large-signal linearity and power-added-efficiency of high-speed graded-channel GaN-based HEMTs at 30 GHz under 64-QAM and 256-QAM at 6V or less operations. These graded-channel GaN HEMTs show the fT of 160 GHz and exhibit a peak power-added-efficiency of 55% at 30 GHz. Under the 64-QAM and 256-QAM, the error-vector-magnitude (EVM) of less than 5 % was measured at the average output power of 16 dBm, about 3 dB backed off from the saturated output power.