Abstract: In this study, we investigated whether and how the molecular weight of fish protein hydrolysate in the plant-protein diets would affect the digestive ability of juvenile turbot (Scophthalmus maximus L). The 68-day experi-ments were conducted on fish with initial weight of (16.050.03) g. Five groups of diets (UF, FPH, RF, PP, and FM) were formulated to be isolipidic, isonitrogenous and isoenergetic. UF (ultrafiltered fish protein hydrolysate), FPH (fish protein hydrolysate) and RF (retentate fish protein hydrolysate) contained 5.4%, 5.5% and 5.5% of protein hydrolysates respectively (10% of total dietary protein). PP contained zero protein hydrolysate. Fish meal consisted of 18% of the diets in UF, FPH, RF, and PP, and it was the sole protein source in the control diet (FM). The special growth rates of fish fed with UF (UF fish), FPH and PP were not significantly different from FM fish (P0.05). There was also no difference in feed efficiency (FE), protein efficiency ratio (PER), or protein productive value (PPV) between UF fish and FM fish (P0.05). However FPH fish, RF fish, and PP fish displayed significantly lower FE, PER and PPV than FM fish did (P0.05). The apparent digestibility coefficients (ADC) for the dry matter and protein of UF, FPH and RF fish were significantly higher than that of PP fish (P0.05), but lower than that of FM fish (P0.05). Between UF, FPH, and RF fish, the ADC for the dry matter and protein of UF was significantly higher than that of the latter two (P0.05). The ADCs for 16 amino acids and taurine could be obviously affected by different dietary treatments (P0.05). For most amino acids the ADC was the highest in FM fish and the lowest in PP fish among all the groups. The ADCs for cysteine and taurine exhibited opposite patterns between the fish protein hydrolysates-containing groups (UF, FPH and RF) and the fish protein hydrolysates-lacking groups (FM and PP). In conclusion, addition of UF and FPH to the high plant pro-tein diets could help increase the growth and feed utilization of the juvenile turbot; the ACD for dry matter, protein, and amino acids could be improved with FPH and UF; compared to FPH and RF, the low molecular weight UF could be more effective to improve the growth. However, it seemed that the fish protein hydrolysate might reduce the ACD for taurine.