In this study, Western blot was used to analyze the expression of insulin like growth factor-1 (IGF-Ⅰ) and its receptor (IGF-ⅠR) proteins during larval development and metamorphosis of Japanese flounder, Paralichthys olivaceus. The results showed that IGF-Ⅰ protein has a higher immunoreactivity at premetamorphosis (17 d) and the onset of metamorphosis (21 d), while lower during metamorphosis. The expression pattern of IGF-ⅠR protein at the onset of metamorphosis was similar to that of IGF-Ⅰ, and but an opposite pattern was observed throughout metamorphosis. These results provide a basis for further study on the physiological significance of the IGF-Ⅰ and its receptor in the Japanese flounder metamorphosis.