Gonadosomatic index, germ cell differentiation, ovarian cycle and first sexual maturity in female Patinopecten yessoensis were studied by histologic and cytologic observations. In the early vitellogenic oocyte, the Golgi complex, mitochondria and endoplasmic reticulum were involved in the formation of lipid droplets. In the late vitellogenic oocyte, exogenous substances, namely, glycogen particles and lipid granular substances appeared in the germinal epithelium passed into the ooplasm through the microvilli of the vitelline envelope. Yolk granules and multivesicular bodies were involved in the formation of proteid yolk granules in the late vitellogenic oocyte. Vitellogenesis occurs by endogenous autosynthesis and exogenous heterosynthesis. The auxiliary cells function as nutritive cells in the formation and development of the previtellogenic and early vitellogenic oocytes in their early stages. Monthly changes in the gonadosomatic index were closely associated with ovarian developmental phases. The annual ovarian cycle of this species can be classified into 5 stages; early active stage (September to October), late active stage (November to February), ripe stage (March to May), spawning stage (April to June) and spent/inactive stage (June to September). The percentage of first sexual maturity was 56.3% in individuals of 61.0–70.9 mm in shell height, and 100% in those >81.0 mm.