In 1980, You & Cheng (1980) suggested that in a dense gas medium, relativistic electrons will produce Čerenkov atomic or molecular emission lines of widths Δλ ≈ 1 – 10Å. The Čerenkov line is broader than a normal emission line and has small redshift (ΔZ
≈ 10–3), so the apparent velocity is about a few hundred km s−1. We refer to this as the ‘Čerenkov redshift’. In 1986, You & Cheng gave simplified formulae for Čerenkov line emission and the mechanism was confirmed by a series of elegant experiments (Xu et al. 1988, 1989). Recent progress in studies of AGNs, both in theory and observation, provide support for the Čerenkov line-emission model of the BLR of AGNs. In this paper, we prove that for a dense gas, if there are enough relativistic electrons, the Čerenkov line emission is strong enough to compare with the observations, and the Čerenkov line emission dominates over the spontaneous emission lines in the optically thick case.