Tunisian acid activated clay was investigated as catalyst in the esterification of stearic acid with ethyl alcohol, carried out in a semi-continuous reactor. Kinetic study shows that the reaction is first order with respect to acid concentration. The activation energy found was 21 kJ/mol. This result suggests that the reaction process was controlled by a diffusional step. A kinetic model, giving the apparent constant rate as a function of temperature and catalyst concentration, has been established. This equation has been then successfully applied for a complex mixture of fatty acid.