Due to the difficulty of p-type doping in β-GaO, NiO/β-GaO heterojunction becomes a promising candidate for fabricating bipolar devices. In this work, we performed a comparative study on the band alignment and electrical properties of the NiO/β-GaO heterojunctions with different β-GaO orientations. NiO thin films were sputtered on three β-GaO substrates with (−2 0 1), (0 0 1) and (0 1 0) orientations and were subsequently fabricated into NiO/β-GaO heterojunction diodes. A type-Ⅱ band alignment between the NiO and β-GaO was identified by the transmittance spectra and X-ray photoelectron spectroscopy (XPS) measurements. The valence band offsets (VBOs) of the NiO/β-GaO heterojunctions on (−2 0 1), (0 0 1) and (0 1 0) substrates were determined to be (2.12 ± 0.06) eV, (2.44 ± 0.07) eV and (2.66 ± 0.07) eV, respectively, and their corresponding conduction band offsets (CBOs) were (1.22 ± 0.06) eV, (1.49 ± 0.07) eV and (1.86 ± 0.07) eV. In addition, the fabricated heterojunction diodes showed good rectification properties with different turn-on voltages, which was in good agreement with the XPS results. The revealed influence of substrate orientations on the properties in NiO/β-GaO heterojunctions were of great importance for the design and optimization of β-GaO-based heterojunction devices.