An eight week feeding trial was conducted to investigate the effects of dietary supplementation of hizikia (Hizikia fusiformis) on growth performance, immune responses and resistance of juvenile olive flounder (Paralichthys olivaceus) to Streptococcus iniae. Four experimental diets (designated as Hiz 0, Hiz 2, Hiz 4 and Hiz 6) were formulated to be isonitrogenous (50% crude protein) and isocaloric (17.2 MJ/kg DM). Hizikia powder was added at 0%, 2%, 4% and 6% in diets Hiz 0, Hiz 2, Hiz 4 and Hiz 6, respectively. Three replicates of fish groups (15 fish/tank) were fed one of the experimental diets. At the end of feeding trial, no significant differences were observed in final body weight, specific growth rate, protein efficiency ratio, feed utilization and feed intake among fish groups fed the experimental diets. However, there was clear trend that the growth performances of fish were improved by the increment of dietary hizikia showing a positive growth effects. Mean phagocytes activated with nitro-blue-tetrazolium were significantly increased with the increment of dietary hizikia. The cumulative mortality was significantly (p