This study investigated the nutritive quality of olive flounder Paralichthys olivaceus fed either moist pellet (MP) or moist pellet mixed with mushroom extract (MPME) for 6 months. There was no significant difference in crude protein or extractive nitrogen in the muscle of flounder fed MP versus MPME (P > 0.05). The total amino acid content in the muscle of flounder fed MP was $15.22{pm}5.24$ g/100 g, compared to $19.90{pm}2.90$ g/100 g for flounder fed MPME. Essential amino acid content was $7.04{pm}2.21$ g/100 g in the muscle of flounder fed MP versus $8.94{pm}2.50$ g/100 g for MPME. Total amino acid content was higher in the muscle of olive flounder fed MPME, while essential amino acid content was higher in flounder fed MP. The ratio of non-essential amino acids to essential amino acids was $0.86{pm}0.07$ for flounder fed MP and $0.81{pm}0.08$ for flounder fed MPME. There was no significant difference in free amino acid content and fatty acid composition. The breaking strength of muscle of olive flounder fed MP was higher ($1.44{pm}0.51;kg/cm^2$) than in flounder fed MPME ($1.29{pm}0.30;kg/cm^2$). There was no evidence that dietary additives, such as mushroom extract, increase growth rate or nutritive quality of olive flounder.