Considerable research exists on self-heating or spontaneous combustion of coal stockpiles from various aspects. The equation for critical temperature of self-ignition was derived from the Frank-Kamenetskii model expressing heat balance between heat generation rate in the centre of a coal pile and heat transfer from the outer surface of the pile. However, critical ignition-temperatures of low-rank coals have not been established for safety criteria in storing and transporting these coal. In this study, experimental apparatus and measurement procedures of thermal diffusivity and internal temperature in the coal pile have been presented to evaluate the critical self-ignition temperature of coal samples. The coal samples tested were low-rank lignite and sub-bituminous. Their critical ignition-temperatures were evaluated based on laboratory temperature measurements of coal piles in cube mesh-boxes of three different sizes (25, 50 and 100 mm side length) placed in hot ambient-air at temperatures ranging from 50 to 140 ºC under standard atmospheric pressure. Analysis of the results has enabled the critical ignition-temperature of the coal sample to be presented as a function of stockpile volume.