SO 2 concentraiton measurement of fl ue was studied with UV differential optical absorption spectroscopy (DOAS) in th is paper.The system consisted of a broadband deuterium lamp,a rotating dispersiv e grating and a photomultiplier tube as a detector.The analyses of the experiment al data indicates that the SO 2 concentration of flue can be accurately measur ed with this method.Other gas components in flue than SO 2 can be separated from their different absorption spectra.The slow variations in the spectral int ensty of the light source or in the transmission of the optical system are also eliminated.