The investigation of mm-sized coal particle fragmentation initiation is of practical interest because of its great importance in the analysis of coal utilization processes such as combustion or gasification in fluidized-bed reactors. Fragmentation experiments were conducted within a drop-tube furnace with eleven kinds of coals of different sizes, and the influences of different factors were discussed. The fragmentation index increased monotonically as the temperature and particle size increase, and the heating rate at different conditions was estimated. No significant correlation was revealed between the fragmentation and the volatile matter, and the thermal stress was deemed to be the main cause of fragmentation. The influences of porosity and tensile strength on fragmentation were more pronounced. Generally, the fragmentation tended to be less extensive as the porosity and tensile strength increased. Accordingly, a parameter consisting of porosity and tensile strength was proposed and defined as the fragmentation resistance number (FRN). The results showed that the larger the FRN is, the more difficult the particles are to fragment; furthermore, the FRN was found to apply well for predicting the fragmentation initiation temperature (FIT) and evaluating fragmentation.