Electron temperature and density are fundamental parameters of ionized gas in HII regions. The electron temperature of ionized gas is thought to infer metallicity, because collisionally excited lines from metals (O and N) are the dominant cooling mechanism in H II regions. However, it is hard to measure electron temperature, especially in UC/HC HII regions with heavy extinction, while radio recombination lines (RRLs) are used to derive electron temperature in such sources. Nevertheless, the two major assumptions, optically thin emission of both RRLs and free-free continuum, and LTE of hydrogen atoms, are incorrect in many cases. I will introduce inverted level populations of hydrogen atoms with both theoretical considerations and observational evidences using RRLs, as well as related uncertainties for electron temperature estimation.
