Hexagonal boron nitride (h-BN) exhibits a dangling bond-free layered structure and ultrawide band gap, which is apt to integrate with other semiconductors to form a heterojunction. Particularly, heterojunction structure is the main impetus for h-BN to broaden the horizon on deep ultraviolet optoelectronic and photovoltaic applications. Here, a series of h-BN/B 1−x Al x N heterojunctions with different Al components were fabricated by radio frequency (RF) magnetron sputtering. The performance of h-BN/B 1−x Al x N heterojunction was measured via I−V characteristic representation. The sample of h-BN/B 0.89 Al 0.11 N heterojunction was the best one due to the high lattice matching. Moreover, a type-II (staggered) band alignment was formed in this heterojunction which was elucidated by X-ray photoelectron spectroscopy (XPS). The calculated valence band offset (VBO) and conduction band offset (CBO) of h-BN/B 0.89 Al 0.11 N are 1.20 and 1.14 eV, respectively. The electronic properties and formation mechanism of h-BN/ B 0.89 Al 0.11 N heterojunction were further studied by density functional theory (DFT) calculation. The existence of a built-in field (E in ) was confirmed, and the E in direction was from the BAlN side to h-BN side. The staggered band alignment was further verified in this heterojunction, and an Al−N covalent bond existed at the interface from calculated results. This work paves a pathway to construct an ultrawide band gap heterojunction for the next-generated photovoltaic application.