Mercury (Hg) is a toxic and ubiquitous trace element in coal. Monitoring its release behavior during coal processing is a challenging problem. This paper presents a method that is capable of online measurement of the dynamic release behavior of elemental Hg (Hg-0) from thermal decomposition of coal. The method couples a temperature-programmed decomposition unit with an atomic fluorescence spectrometry detector (TPD-AFS). The AFS signal can be converted to Hg content for quantitative analysis through calibration. The main advantage of the TPD-AFS system is that it can provide either real-time or accumulated data. It was confirmed that the amount of Hg-0 determined by this method agreed well with that by the Ontario-Hydro method, and the results are reproducible with a high accuracy. The method can be used to characterize the release behavior of Hg in coals upon heating and may be used to identify the forms of occurrence of Hg in coals.