The mercury release from coal gangue and coal slime has attracted wide interest, because of the utilization of coal gangue and coal slime in some power plants. In order to develop the Hg control technology, it is necessary to gain the information on the Hg release during thermal treatment of coal gangue and coal slime. This paper presents a study about Hg release during thermal treatment of two coal gangues and coal slimes under air and N-2 atmosphere through the online atomic fluorescence spectroscopy aided with the Ontario Hydro Method. The result shows that the release profiles of Hg for the two coal gangues and two coal slimes are similar under the same atmosphere, respectively. Most of the Hg is released at <650 degrees C under a N-2 atmosphere and at <500-550 degrees C under air for all of the samples. The release ratio of Hg2+ is much lower than that of Hg-0 for each sample. The release ratio of Hg-0 released under N-2 is more than that under air. Because of the high volatility of Hg, the release ratio of total Hg is much higher than the corresponding volatile yield in any case investigated in this study. Meanwhile, the Hg-0 release profiles for the samples show two or three peaks under N-2 atmosphere, suggesting that different modes of occurrence of mercury are presented in the coal gangues and coal slimes.