Using the result of model-solid theory,we have obtained the relationship between bandgap and strain of Si1-xGex alloy on Si(100) subs trate with x<085 . It was shown that the deviation between the bandgap of strained SiGe and relax ed SiGe is proportional to the strain. According to the theoretical result, a no vel method was suggested to determine the strain state of SiGe/Si through measur ing the bandgap. The strain in the SiGe/Si multi-quantum wells was measured using the new method and the results had good agreement with that from XRD measurement.