The Oriental armyworm, Mythimna separata Walker (Lepidoptera: Noctuidae), is a major polyphagous pest with a wide range of hosts in Asia and Oceania. Scanning electron microscopy was used to further elucidate the function of the antennal sensilla and their type, number, and distribution in M. separata. The antenna of M. separata is filamentous. On each three-segmented antenna, from the basal scape, there is a pedicel and a flagellum composed of 70–80 subsegments. Eight morphologically distinct types of sensilla were found on the antennae, including sensilla trichodea, two types of sensilla chaetica, sensilla basiconica, sensilla styloconica, sensilla coeloconica, sensilla squamiformia, sensilla auricillica and Böhm bristles. Sensilla auricillica were found on the antennae of both sexes of M. separata for the first time. The sensilla auricillica in females were located on flagella F23–F44, while they were located on flagella F5–F80 in males.