The band offset between strained GexSi1-x and unstrained Si is mainly due to the valence band offset ΔEv. The thermal emission energy from a quantum well is related to the appropriate band offset. The single quantum well (p type Si/Ge0.25Si0.75/Si) samples were grown by molecular beam epitaxy (MBE), the width of quantum well is 15nm. Deep leve transient spectroscopy (DLTS) measurement have been used to study the valence band offset. After considering the band bending due to the electric field and the first subband energy, the valence band offset ΔEv of the single quantum well was estimated as about 0.19eV, it is in reasonable agreement with theoretical results. The same and difference of emission and capture processess between quantum well and deep level defects were discussed.