This study was conducted to provide the reproductive biological information and basic data on the artificial sex control of Haliotis discus hannai. The morphological sex differentiation process of H. discus hannai could be classified into following five phases: 1) formation of gonad outer membrane (FGOM) (≤ SL 10.0 ± 1.0 mm), 2) primordial germ cells (PGCs) appearance in the connective tissue between intestine and hepatopancreas (PAC), and formation of gonadal cavity (FGC) (SL 15.0 ± 2.0 mm), 3) PGCs appearance in the epithelial layer of gonadal cavity (PAG) (SL 18.0 ± 2.0 mm), 4) formation of gametogenic follicle and appearance of early oocytes and spermatogonia (FGOC) (SL 21.0 ± 2.0 mm), 5) morphological sex differentiation (MSD) (≥ SL 23.0 ± 2.0 mm). From histological analysis sex differentiation rate in SL 24.1-25.0 mm of H. discus hannai was 90.0% and sex ratio (female : male) was 1:0.8.