[Objectives]  Magnetic particles are important for some organisms to sense changes in the geomagnetic field. They are also one of the important geomagnetic orientation mechanisms used by insects during long distance nocturnal migration. In this study, the locations of magnetic particles in adult oriental armyworms, Mythimna separata, a nocturnal migratory insect pest in China, were identified. [Methods]  SQUID was used to detect magnetic particles in the heads and abdomens of adult M. separata, and a standard BX61 microscope and Prussian blue stain were used to observe the distribution of magnetic particles in the abdomen. [Results]  The abdomen was weakly magnetic relative to the hysteresis loop in the head, indicating that there may be magnetic particles in the abdomen of adult M. separata. Furthermore, microscopic observation revealed obvious Prussian blue precipitation in the abdomen. These results demonstrate that there are ferromagnetic particles in the abdomen of adult M. separata. [Conclution]  The adult abdomen could be important for sensing the geomagnetic field and for geomagnetic orientation in this species.