Melanism caused by hypermelanosis on the blind side is common in artificial breeding of Japanese flounder(Paralichthys olivaceus).At present, the mechanism of melanism is not clear. This research found that no melanophore are found in the normal skin on blind side, adult melanophore appeared on the melanized blind side of hypermelanosis specimens and the larval-type melanophore were unexpectedly found, the density of melanophore is significantly lower than normal skin on the ocular side; ctenoid scales appear on hypermelanosis skin, and normal skin on the ocular side is still cycloid scales; The number of ctenii on the ocular side was all above 5 ctenii, while the number of ctenii on the melanized blind side was all below 5 ctenii, showing a significant difference. This research found that the density of melanophore was significantly correlated with the number of ctenii. The higher the density of melanophore, the greater number of ctenii. It is presumed that the formation of scales is based on the increase of the density of melanophore. This experiment examined the expression of RALDHs(RALDH1, RALDH2, RALDH3) in the melanized skin and normal skin on the blind side. The results showed that the expression of RALDH1 and RALDH3 in the melanized skin was higher than that in the normal skin, and the difference was significant (P<0.05). The expression of RALDH2 was not significantly different between the two part. The results suggest that there is a correlation between melanism and ctenoid scale in Japanese flounder, and may be related to the synthesis of retinoic acid.