In order to understand the electronic structure of GaN/AlN heterojunction deeply,it is calculated by the density functional theory based on the first principle plane wave pseudopotential method,via the super cell model.The direct calculation of the GaN/AlN super cell demonstrates that the GaN/AlN heterojunction is homotype and changes abruptly at the interface,besides,the valence-band offsets of GaN/AlN is 0.62 eV,which is very close to the experimental result.Additionally,it can be concluded that the super cell calculation has more advantages than the other common indirect calculation of heterojunction band offsets,such as the average bond energy,the average potential and the core state.The super cell calculation supplies more information of the GaN/AlN heterojunction interface while the reference energy band methods can not.Nevertheless,band-offsets parameters obtained by the other indirect methods are also reasonable compared with the experimental value.