Trans-Cinnamic acid and its derivatives were investigated for the a-glucosidase inhibitiory activity. 4-methoxy-trans-cinnamic acid and 4-methoxy-trans-cinnamic acid ethyl ester showed the highest potent inhibitory activity among those of trans-cinnamic acid derivatives. The presence of substituents at 4-position in trans-cinnamic acid derivatives. The presence of substituents at 4-position in trans-cinnamic acid altered the a-glucosidase inhibitory activity. Increasing of bulkiness and the chain length of 4-alkoxy substituents as well as the increasing of the electron withdrawing group have been shown to decrease the inhibitory activity. 4-methoxy-trans-cinnamic acid was a non-competitive inhibitor for a-glucosidase, whereas. 4-methoxy-trans-cinnamic acid ethyl ester was a competitive inhibitor. These results indicated that trans-cinnamic acid derivatives could be classified as a new group of a-glucosidase inhibitors.