Transportating and stockpiling low-rank coals are puts them under risk of spontaneous combustion, because they are very easy to get self heating ignition after drying or removing moisture due to high moisture and oxygen contents. In this study, the modified concept of Equivalent Oxidation Exposure-time (EOE-Moisture time) has been presented for row-rank coals. The equations for the EOE-Moisture time have been formulated by considering moisture content and oxidation capacity in low temperature range to predict the heat generating rate as functions of coal moisture saturation and oxygen concentration in the pile. Numerical simulations were carried out by applying the EOE-Moisture time on self-heating of low rank coal by changing the coal stockpile size. It has been shown based on the numerical simulations that the temperature rising rate of the pile is increased with lower initial moisture saturation of low rank coal. It has been shown that coal including a large percent of moisture is easy to increase temperature and has a smaller critical diameter of coal stockpile size compared with coals with low moisture.