We investigated the effects of rearing water temperatures during sex differentiation period on sex ratios in olive flounder, Paralichthys olivaceus. Control and gynogenetic diploid juvenile flounder reared at water temperature of 18, 21, 24 and $27^{circ}C$ for 65 days from 35 to 100 days after hatching. Fish were sampled to examine sex ratios at 195 or 260 days after hatching. Female ratios of control and gynogenetic diploid flounder declined rapidly as water temperature increased. Sex ratio of gynogenetic diploid reared at $27^{circ}C$ was very close to 1 : 1 ratio (P<0.01), The survival rates of control and gynogenetic diploid reared at $27^{circ}C$ were different from other water temperature groups. The growth of body weight of control and gynogenetic diploid reared at $18^{circ}C$ were different from other water temperature groups.