A new process for synthesis of ( 2S,3S)-2-ethyl-3-methylvaleramide has been developed via the key step of a diastereoselective alkylation reaction using non-cross-linked polystyrene (NCPS) supported ( 4S)-2-phenylimino-2-oxazolidine as a chiral auxiliary. This method is efficient with the target product obtained in 99.24% ee and 38.4% overall yield and the chiral auxiliaries can be recovered quantitatively by simple filtration.