In this paper, selective area growth (SAG) of a-plane GaN/AlGaN double heterostructure (DH) has been performed on r-plane sapphire substrates by using mixed-source hydride vapor phase epitaxy (HVPE) with multi-sliding boat system. The SAG-GaN/AlGaN DH is consisted of a GaN buffer layer, a Te-doped AlGaN cladding layer, a GaN active layer, a Mg-doped AlGaN cladding layer, and a Mg-doped GaN capping layer. The electroluminescence (EL) characteristics show an emission peak wavelength of 438 nm with a full width at half maximum (FWHM) of approximately 0.64 eV at 20 mA. The I-V measurements show that the turn-on voltage of the SAG-GaN/AlGaN DH is 3.42 V at room temperature. We found that the mixed-source HVPE method with a multi-sliding boat system is one of promising growth methods for III-Nitride LEDs.