In this work, coal gasification in molten blast furnace (BF) slag was performed at atmospheric pressure in a molten bath reactor in the temperature range from 1573 to 1773 K. The effects of coal samples and coal granularities on the gasification reaction rate, carbon conversion, and gas composition were studied, and the kinetic mechanism function was calculated. The results show that the method using coal gasification to recover sensible heat of molten BF slag is possible and effective. The coal gasification using molten BF slag as a carrier has wide adaptability on coal granularities and coal samples. It is effective to gasify lean coal. In this study, the gasification reaction rate of Shennan (SN) coal that represents lean coals is high and almost equal to that of Datong (DT) and Fuxin (FX) coals. The carbon conversion of all types of coal can become 0.99. Under the action of molten slag, the tar and hydrocarbon decompose fully; the content of other hydrocarbons in synthesis gas is almost zero. The reaction rates using different coal granularities increase in the order of 0.177 < 0.125 < 0.147 mm, but the difference is little. The calorific capacity of synthesis gas under different conditions is almost equal and can become 10 000 kJ/m(3). The mechanism functions are different before and after the carbon conversion rate of 0.5. The mechanism function is shown as follows: f(a) = {3(1 - a)[-ln(1 - a)](2/3) a <= 0.5 2(1-a)[-ln(1-a)](1/2) a > 0.5