Abstract: In the last decade, the increasing demand, price and world supply fluctuations of fish meal has emphasized the need to look for alternative protein sources in aquafeeds. Increased use of protein of plant origin and better use of available marine protein sources are important issues in this respect. The total volumes of by-products from the seafood industry are significant and could become valuable ingredients in feed for carnivorous fish species. Processing of fish hydrolysate from the by-products is applicable and these products are relevant as ingredients for fish feed. The present experiment was carried out to evaluate fish protein hydrolysate as feed ingredient in high plant protein diets for Japanese flounder (Paralichthys olivaceus). Fish protein hydrolysate was produced from Pollack (Pollacbius pollacbius) by enzymatic treatment. The fish hydrolysate was size fractionated by filtration. Fish protein hydrolysate product A (FPH-A) was the permeate sample after filtration. Fish protein hydrolysate product B (FPH-B) was the non-filtrated sample. Japanese flounder juvenile were fed with diets containing 1.2% and 3.7% fish protein hydrolysate in a continuous flow system. The growth experiment lasted for 60 days. The results showed that fish protein hydrolysate improved the growth of Japanese flounder. Specific growth rates of fish fed with 3.7% FPH-A was the highest. There were no significant differences in feeding rate of flounder. Crude protein composition of fish fed with 3.7% FPH-A was significantly higher than fish fed with control diet (fish meal). Fish protein hydrolysate significantly improved protein digestibility, protein retention and protein utilization of flounder juvenile. Fish fed with 3.7% FPH-A had the highest protein digestibility, protein retention and protein utilization. In conclusion, small molecular weight compounds from fish protein hydrolysate may successfully be used as protein source in high plant protein diets for Japanese flounder in exchange of fish meal.