Large signal RF and DC performance of n-type 6H-SiC and p-type diamond MESFETs has been simulated at various operating temperatures by a large-signal RF simulator using the harmonic balance technique and the two-dimensional device simulator, PISCES-IIB. The RF performance of SiC MESFET is predicted to be optimal in a temperature range slightly higher than room temperature. At room temperature the simulated SiC MESFET exhibits an output power of 3.5 W/mm for an operating frequency of 8 GHz with 16.5 dB gain and 44% power-added efficiency at 24 dBm input power. In contrast to the SiC MESFET, the RF performance of the diamond MESFET is improved with temperature, but the current level is much lower than that in SiC in the entire temperature region investigated. The very different temperature dependencies of DC and RF performance in SiC and diamond MESFETs are attributed to the significant difference in the dopant ionization energies in SiC and diamond.