The gamma-ray irradiation effects on channel mobility (mu) and threshold voltage (V-T) for n-channel 6H-SiC metal-oxide-semiconductor field effect transistors (MOSFETs) were studied. The gate oxide of the MOSFETs was fabricated using pyrogenic oxidation and subsequent annealing in steam at 800 degreesC or in H-2 at 700 degreesC. The value of mu for the MOSFETs whose gate oxide was annealed in steam and in H-2 is not changed up to 180 and 60 kGy(SiO2), respectively. By irradiation at 530 kGy(SiO2), the change of V-T for the MOSFETs whose gate oxide was annealed in steam and in H-2 is 0.6 V (from 2.7 to 3.3 V) and 2.2 V (from 0.9 to 3.1 V), respectively.