The β-Ga ₂O ₃has received much attention in the field of power and radio frequency electronics, due to an ultrawide bandgap energy of ~4.9 eV and a high breakdown field strength of ~8 MV/cm (Poncé et al. 2020 Phys. Rev. Res. 2033102 ). The in-plane lattice mismatch of 2.4% between the ( $ \bar 201 $ ) plane of β-Ga ₂O ₃and the (0002) plane of wurtzite AlN is beneficial to the formation of an AlN/β-Ga ₂O ₃heterostructure (Sun et al. 2017 Appl. Phys. Lett. 111162105 ), which is a potential candidate for β-Ga ₂O ₃-based high electron mobility transistors (HEMTs). In this study, the Schrödinger-Poisson equations are solved to calculate the AlN/β-Ga ₂O ₃conduction band profile and the two-dimensional electron gas(2DEG) sheet density, based on the supposition that the 2DEG originates from door-like surface states distributed evenly below the AlN conduction band. The main scattering mechanisms in AlN/β-Ga ₂O ₃heterostructures, i.e. the ionized impurity scattering, interface roughness scattering, acoustic deformation-potential scattering, and polar optical phonon scattering, are investigated by using the Boltzmann transport theory. Besides, the relative importance of different scattering mechanisms is evaluated. The results show that at room temperature, the 2DEG sheet density increases with the augment of AlN thickness, and reaches 1.0×10 ¹³cm ^–2at an AlN thickness of 6 nm. With the increase of the 2DEG sheet density, the ionized impurity scattering limited mobility increases, but other scattering mechanisms limited mobilities decrease. The interface roughness scattering dominates the mobility at low temperature and moderate temperature ( T< 148 K), and the polar optical phonon scattering dominates the mobility at temperatures above 148 K. The room-temperature mobility is 368.6 cm ²/(V·s) for the AlN/β-Ga ₂O ₃heterostructure with an AlN thickness of 6 nm.