A quantum theory of Cerenkov radiation in gas medium is estiblished by means of the first-order perturbation of quantum feild theory. The results show that there are some characteristics different from Cerenkov radiation of liquid and solid. The main features of this Cerenkov radiation are as follows: (1) Line emission, asymmetric profile and small red shift; (2) The radiation angle is independent of frequency and decreases along with increasing of the speed of incident charged particles: (3) No speed threshold of the charged particles exists; (4) No matter whether and atom is at ground state or at excited state, the pro-bility of Cerenkov radiation is identical. The theoretical predictions coincide with the experiments.