Abstract: We investigated the annual developmental rule of hermaphroditic gland of Onchidium struma from Jiangsu, Zhejiang and Shanghai. The results indicate that O.struma is a hermaphrodite which often becomes a functional male before its female organs develop. The smallest individual is 2.81 cm in length and 1.81 g in weight. The development of hermaphroditic gland has a cycle per year, which is divided into proliferation, spermatogenesis, ova and sperm, exporting, and resting stages. Specifically, exporting stage is divided into sperm exporting, simultaneous exporting of synchronous occurence of ova and sperm, and ova exporting, indicating a heterochrony of spermatogenesis and oogenesis. The male reproductive cells have spermatogonia, primary spermatocyte, secondary spermatocyte, spermatid and sperm. The female ones have oogonium, previtellogenic oocyte, vitellogenic oocyte, postvitellogenic oocyte and maturation. The annual change of gonadosomatic index (GSI) shows a unimodal distribution whose peak value occurs during July and September. The albumin gland index (AI) and vitelline gland index (VI) are correlated with GSI. AI changes along with GSI and has three peak values during GSIs change, which occur in June, August and December, respectively.