Facilitation of fear extinction learning is one of the desirable actions for the drug that has efficacy to enhance cognitive behavioral therapy in the treatment for fear-related psychological diseases, such as posttraumatic stress disorder (PTSD). We previously reported that selective agonists of δ-opioid receptor (DOP), KNT-127, facilitates extinction learning of contextual fear. However, it’s site of action in the brain remains unknown. Therefore, we investigated brain regions which mediate the action of KNT-127 on fear extinction contextual fear conditioning in mice. Male C57BL/6J mice were used. On day 1, the mice were conditioned with 8 foot-shocks (0.8 mA, 1s, 30s interval) in the conditioning chamber. On day 2, mice were re-exposed to the chamber for 6 min as an extinction training (re-exposure 1). KNT-127 was microinjected into the amygdala and hippocampus 30 min before re-exposure 1. On day 3, mice were re-exposed to the chamber for 6 min as a memory testing (re-exposure 2). Microinjection of KNT-127 (0.125μg/ml) in the basolateral nucleus of the amygdala (BLA), but not dorsal and ventral part of hippocampal CA1, significantly reduced freezing responses both in re-exposures 1 and 2 when compared with those of control mice. These results suggested that KNT-127 exerts extinction-facilitating effect via DOPs in the BLA.