The β-Ga₂O₃ has 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. 2 033102). The in-plane lattice mismatch of 2.4% between the ( \bar 201 ) plane of β-Ga₂O₃ and the (0002) plane of wurtzite AlN is beneficial to the formation of an AlN/β-Ga₂O₃ heterostructure (Sun et al. 2017 Appl. Phys. Lett. 111 162105), which is a potential candidate for β-Ga₂O₃-based high electron mobility transistors (HEMTs). In this study, the Schrödinger-Poisson equations are solved to calculate the AlN/β-Ga₂O₃ conduction 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-Ga₂O₃ heterostructures, 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 1.0×10¹³ cm^–2 at 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 cm²/(V·s) for the AlN/β-Ga₂O₃ heterostructure with an AlN thickness of 6 nm.