In this paper, we present a comparison of the direct-current (DC) and radio-frequency (RF) breakdown behaviours of representative wurtzite- and zinc-blende-phase GaN MESFET structures based on a theoretical analysis. The calculations are made using a full-band ensemble Monte Carlo simulation that includes a numerical formulation of the impact ionization transition rate. Calculations of both the DC and RF breakdown voltages are presented for submicron MESFET devices made from either wurtzite- or zinc-blende-phase GaN. The devices are otherwise identical. It is found that the DC breakdown voltage in the wurtzite-phase GaN MESFET is significantly larger than that in the zinc-blende-phase device. This is due to the fact that electron heating occurs more rapidly within the zinc-blende phase than the wurtzite phase of GaN. As a consequence, avalanche breakdown occurs at higher applied field strengths and voltages in the wurtzite phase than in the zinc-blende phase of GaN. It is further found that the RF breakdown voltage of the devices increases with increasing frequency of the applied large-signal RF excitation.