AlGaN/GaN HFETs were fabricated around micropipes and on a domain boundary in a semi-insulating silicon carbide (SI-SiC) substrate and the DC characteristics of the fabricated devices were measured. Devices around micropipe showed no pinch-off or large gate leakage. The devices on the domain boundaries showed no degradation in the performances, even though an X-ray topographic analysis indicated that crystal imperfections, due to the defects, propagated to the GaN layer across the hetero interface. Based on these results, we concluded that micropipe degrades the DC characteristics and that the domain boundary does not affect the DC characteristics. From Raman analysis on the devices around the micropipes, these degradations could be attributed to the free carriers introduced into the GaN crystal by the micropipes.