Although Li-ion batteries (LIBs) have had great success in portable electronic devices and electrical vehicles, the improvement of the performances has received intensive attention. Generally, doping is an effective method to modify the battery performance, such as cycling performance. Appropriate doping can effectively reduce the structural deformation of electrode materials during charging and discharging, thus improving the cycling performace of LIBs. Because of the large radius, large charge and strong self-polarization ability of rare earth ions, rare earth element is a promising candidate for doping modification. Motivated by this, we study the structural, electronic and ionic diffusion properties of rare-earth-doped cathode material Li₂MnO₃ by using first-principles calculations based on density functional theory as implemented in Vienna ab initio simulation package. After the doping of rare earth elements (La, Ce, Pr, Sm), the lattice constants and cell volumes increase with respect to the undoped one. The cell volume of La-doped Li₂MnO₃ has the biggest change, while the cell volume of Sm-doped one has the smallest change. Due to the different ionic valence states, the electronic structures of the doped Li₂MnO₃ are various. La-doped Li₂MnO₃ exhibits metallic characteristic, whereas Ce-, Pr-, and Sm-doped structures are semiconducting with smaller band gap than that of the undoped case. The Li diffusion energy barrier in Li₂MnO₃ shows complicated variation when the La and Ce are doped. At the sites far away from the rare earth ions, the Li diffusion barriers are lower than that of undoped one. The reason is that the diffusion channels, which are determined by the distance between neighboring O-layers, are enlarged due to the implantment of rare earth ions. However, the situations are various at the sites close to the rare earth ions. The Li diffusion barriers increase essentially when Li ions diffuse from the nearest sites to rare earth ions. Such a result is closely related to the huge changes of local structures around the rare earth ions. In addition, the effect of La doping on the Li diffusion barrier is more obvious than that of Ce doping, which is due to the local structure change around rare earth ions.