High and low dose-rate radiation damage effects of P-type cap and cascode structure gallium nitride power devices were investigated. Experimental results showed that neither P-type cap nor cascode structure gallium nitride power device had Enhanced Low Dose Rate Sensitivity effect (ELDRS). The electrical parameters of cascode structure gallium nitride power devices degraded more obviously after total ionizing dose irradiation. The P-type cap structure of GaN power devices had strong resistance to total ionizing dose. The mechanism of the degradation in electrical parameters after irradiation was analyzed. The experimental results of this study provided a useful reference for comprehensively evaluating GaN power devices utilized for space applications.