This study was conducted to investigate the effects of brown seaweed (Undaria pinnatifida) by-product and seaweed fusiforme (Hizikia fusiformis) by-product supplementation on growth performance and blood profiles including serum immunoglobulin (Ig) in broilers. Fermentation of seaweeds was conducted by Bacillus subtilis and Aspergillus oryzae. In a 5-wk feeding trial, 750 one-d-old broiler chicks were divided into 5 groups, and were assigned to the control diet or experimental diets including control+0.5% brown seaweed (BS) by-product, control+0.5% seaweed fusiforme (SF) by-product, control+0.5% fermented brown seaweed (FBS) by-product, and control+0.5% fermented seaweed fusiforme (FSF) by-product. As a consequence, body weight gain (BWG) and gain:feed of seaweed by-product groups were clearly higher, when compared to those of control diet group from d 18 to 35 and the entire experimental period (p<0.05). In mortality rate, seaweed by-product groups were significantly lower when compared to control diet group during entire experimental period (p<0.05). However, Feed Intake of experimental diets group was not different from that of the control group during the entire experimental period. Whereas, Feed Intake of fermented seaweed by-product groups was lower than that of non-fermented seaweed groups (p<0.05). Total organ weights, lipids, and glutamic oxalacetic transaminase (GOT) of all treatment groups were not different from those of control group. However, glutamic pyruvate transaminase (GPT) of all treatment groups was higher than that of control group at d 17 (p<0.05). In case of serum Igs concentration, the concentration of IgA antibody in BS, SF, FSF treatment groups was significantly higher than in control group at d 35 (p<0.01). IgA concentration in FBS supplementation groups was negligibly decreased when compared to the control group. IgM concentration in the serums of all treatment groups was significantly higher than in control group (p<0.05) and in fermented seaweed by-product groups were much higher than in non-fermented seaweed groups (p<0.05). On the other hand, IgG concentrations in all treatment groups were lower than in control group (p<0.05). Taken together, our results suggest that by-product dietary supplementation of BS, SF, FBS, and FSF in poultry may provide positive effects of growth performance and immune response.