A bubble column reactor has been developed to produce fatty acids methyl esters (biodiesel) by blowing superheated methanol continuously into vegetable oils without using any cat alysts at atmospheric pressure. Two types of reactions exist in this method for methyl esters formation; transesterification of triglycerides and esterification of fatty acids. A kinetic study for the transesterification of triglyceride has been studied at 250-290 degrees C. However, performance of the bubble column reactor for methyl esterifcation of free fatty acids has not yet been revealed. Therefore, we studied effects of reaction temperatures (200, 225, and 250 degrees C) on the composition of reaction product, conversion of the reaction and the reaction rate constant under a semi-batch mode of operation with oleic acids as free fatty acids. The activation energy and the frequency factor values of the methyl esterification reaction obtained in this experiment were 24.8 kJ/mol and 2.9, respectively. Compared with the obtained results of the transesterification of triglycerides, the reaction rate of methyl esterification of free fatty acids at the same reaction temperature (250 degrees C) was higher than the former, but the quality of the product (the methyl ester content in the gaseous product) was lower.