Solvent-free domino acylation/cyclization reactions between fatty acid esters and o-phenylenediamine mediated by immobilized lipase from Mucor miehei (MML) were found to be an efficient way in the synthesis of 2-alkylbenzimidazole. Compared with other substrates, methyl fatty acid esters with moderated chain length exhibited best activity with yield up to 95%. The mechanism of the domino process was