This study investigates the effects of temperature gradient and coal particle size on the critical self-ignition temperature TCSIT of a coal pile packed with low-rank coal using the wire-mesh basket (WMB) test to estimate TCSIT based on the Frank–Kamenetskii equation. The values of TCSIT, the temperature gradient and the apparent activation energy of different coal pile volumes packed with coal particles of different sizes are measured. The supercriticality or subcriticality of the coal is assessed using a non-dimensional index IHR based on the temperature gradient at the temperature cross-point between coal and ambient temperatures for coal piles with various volumes and particle sizes. The critical value IHRC at the boundary between supercriticality and subcriticality is determined as a function of pile volume. The coal status of supercritical or subcritical can be separated by critical value of IHR function of pile volume. Quantitative effects of coal particle size on TCSIT of coal piles are measured for constant pile volume. It can be concluded that a pile packed with smaller coal particles is more likely to undergo spontaneous combustion, while the chemical activation energy is not sensitive to coal particle size. Finally, the effect of coal particle size on TCSIT is represented by the inclusion of an extra term in the equation giving TCSIT for a coal pile.