Abstract: A feeding trial was conducted to investigate the effects of soybean oligosaccharides (SBOS) on feeding rate, growth and metabolize enzyme activities of juvenile Japanese flounder (Paralichthys olivaceus) 1Four isonitrogenous and isocaloric practical diets (crude protein 49%, gross energy 19 kJ/g) including only fish meal (FM) or soy protein isolate (SPI) as a sole protein source with (Diets FMO and SPI O) orwithout (Diets FM and SPI) supplemented SBOS (stachy-ose, 2.61%; raffinose, 0.61%) were formulated1Each diet was randomly designed to triplicate aquaria stocked with 30 fish (initial body weight 1.93 ± 0.02g) each1Fish were maintained in a flow-through system with water temperature ran-ging from 22.0℃ to 25.0℃ and fed twice (08:00 and 16:00) daily to apparent satiation for 8 weeks1Dietary SBOS sup-plementation did not significantly affect the feeding rate in the first two-week (p >0.05), but significantly decreased the gross feeding rate of fish fed FM-based diets, increased the gross feeding rate of fish fed SPI-based diets (p p >0.05) 1However, dietary SBOS supplementation generally increased the SGR, FER and PER of fish fed SPI-based diets, and the difference in SGR was sig-nificant (p p p >0.05) 1SBOS supplementation significantly increased the activities ofALT and LDH in plasma (p p >0.05) 1At the same time, supplemental SBOS significantly increased the activities ofAST and ALT in liver (p p >0.05) 1The urinary nitrogen and free amino acid con-centration in plasma of fish were not affected by supplemental SBOS regardless of dietary protein source (p >0.05). These results indicate that there are different effects of SBOS on growth performance of Japanese flounder fed different protein source based diets; dietary SBOS supplementation generally depresses the growth of Japanese flounder fed FM-based diets, but increases that of fish fed SPI-based diets.