Ultraviolet (UV) electroluminescence (EL) of n-ZnO:Al (AZO)/i-layer/n-GaN heterojunctions with different intrinsic layers has been obtained. Rectifying behavior and EL spectra of the heterojunctions are investigated at room temperature. Under positive voltage, a dominant UV emission peak around ∼370nm is observed for both AZO/i-ZnO/n-GaN and AZO/i-MgO/n-GaN heterojunctions. Nevertheless, the UV emission peak intensity of AZO/i-MgO/n-GaN heterojunction is much stronger than that of AZO/i-ZnO/n-GaN heterojunction at the same voltage. The threshold voltage of AZO/i-MgO/n-GaN heterostructured device is as low as 2.3V. The difference of EL spectra and the emission mechanism in these devices are discussed.