The low solubility of L-methionine and low activity of enzyme are the major hurdles during L-methionine production by the enzymatic conversion approach. In this study, we investigated various ionic liquids (ILs) as additives for the enzyme-catalyzed production of L-methionine from O-acetyl L-homoserine and methyl mercaptan. Among the ILs evaluated, we found that tetraalkylammonium hydroxide ILs enhanced the solubility of L-methionine as well as the activity of the enzyme. Methionine solubility decreased with increasing alkyl chain length but increased with increasing IL concentration. L-methionine could be dissolved up to 232 g/L in 10% tetrarnethylammonium hydroxide solution. The enzyme O-acetylhomoserine aminocarboxypropyltransferase reached its maximum activity when the IL concentration was 2.5% (3 times higher than that without ILs) and significantly decreased with increasing IL concentration. The stability of the enzyme also decreased rapidly after 2 h of incubation regardless of the presence or absence of ILs. Nevertheless, 74 g/L of L-methionine could be produced in a reaction media containing 2.5% tetraethylammonium hydroxide compared to 35 g/L of L-methionine obtained in a reaction system without ILs.