The esterification of oleic acid with methanol as model reaction was carried out on behalf of the main component of fatty acids (FFAs) in waste frying oils, for the purpose of biodiesel synthesis,catalyzed by 1-( 4-sulfonic acid) butyl-3-methylimidazolium hydrogen sulfate([BSMim] HSO₄).The highest yield of methyl oleate in 97. 5% could be obtained under the optimal condition. T he isolation of product esters could be accomplised through simple decantation from the biphasic system attributed to the immiscibility of [BSMim] HSO4 with organic esters. Four times reuse of ionic liquid without dramatic decrease in ester yield demonstrated the good stability and operability of [BSMim]HSO₄.