OBJECTIVE To study the decomposable compound of oxazepam.METHODS Destroy-tests of strong acid,strong alkali,highlight,heat and oxygen were used for oxazepam,then the possible decomposable compounds wre elucidated by HPLC under different mobile phases.The mutual decomposable compound under thermal destruction-test was isolated and prepared by Preparative Thin-layer Chromatogram(PTLC)and its structure was elucidated by UV,IR,¹H-NMR,¹³C-NMR,HMBC,HMQC,ESI-MS.RESULTS One decomposable compound(6-Chloro-4-phenyl-quinazoline-2-carbaldehyde)was isolated and identified as the related substance of oxazepam.CONCLUSION The compound was reported in domestic literature for the first time,providing reference for the improvement of oxazepam standard of Chinese pharmacopoeia.