Three coals of Yangquan anthracite (YQ), Huainan bituminous coal with a medium amount of volatile matter and Shenfu bituminous coal with a high amount of volatile matter (SF) are used for the investigation of less combustible constituents (LCC). It has been concluded that some less combustible constituents exist in coals by means of centrifugal separation (CS) with gravity liquids and thermogravimetric analysis (TG) with a Du Pont-2100 thermal balance. The constituents always lie in the separation fraction with the specific density, which differs from each other in various coals. The LCC in coals always give a lower value of the maximum combustion rate, and also lower kinetic parameters, like activation energy (E) and apparent pre-exponential factor (k(0)).