Based on an ab initio pseudopotentials band structure method,a theoretical approach of tak-ing the average-band energy as a energy reference is suggested to determine the valence-band offsets at Si/Ge heterojunctions under three different strain conditions:using Si as a substrate,using Ge as a substrate and Si-layer,Ge-layer deforming freely.The results are 0.731eV,0.243eV and 0.521eV respectively,in good agreement with relevant experimental values.