Gonadal development and reproductive cycle of the North Pacific seastar, Asterias amurensis captured from the Gosung, Gyeongsangnamdo, between November 2003 and February 2005, was investigated monthly changes of gonadosomatic index(GSI), gonadal development and oocyte size-frequency distribution. Monthly changes of GSI values showed similar trends in female and male. GSI values were reached the maximum in March($3.88{pm}3.04$ in female, $0.87{pm}0.57$ in male), and then gradually decreased. Based on the monthly changes of GSI, histological observation of gonadal development, reproductive cycle was divided into following successive stage: growing stage(October to January), mature stage(February to March), spent stage(March to April), degeneration and resorption stage(April to May), recovery stage(July to September). based on these result, this species seemed to have a synchronous oocyte development and one spawning season a year.