Cerenkov effect in gas medium is discussed. It is pointed out that the relativistic electrons with isotropic distribution of velocity, when moving through a dense gas, will produce the broad atomic emission-lines with a remarkably asymmetric profile, the continuum slope on its long wave side being quite flat and that on the short wave side very steep.Calculation for the hydrogen atom shows that the width of Lα line may be larger than 60? as the gas density reaches N = 5×1020cm-3. According to this radiation mec-. hanism, the emission-line spectra of QSOs can be interpreted satisfactorily.