Fucoidan, a water-soluble heteropolysaccharide that contains fucose and sulfate groups which found in the cellular wall of brown algae which in this study, the effects of its various levels on the non-specific immune response, blood biochemical factors and antibacterial activity of serum in rainbow trout (Oncorhynchus mykiss), with mean weight of 74.8 ± 70 g for 8 weeks were investigated. The treatments consisted of 0 (control), 0.1, 0.5, 1 and 2% of fucoidan per kg of diet in the form of five treatments, three replications were performed in 100 liters tanks. Based on the findings of this study, there were no significant different among different groups of RBC indices such as RBC, HB, HCT, MCV, MCHC and MCH (p> 0.05), while the number of WBC was significantly different among different groups (p<0.05). TP, HDL-C, LDL-C were significantly affected by diets containing fucoidan (p<0.05). There was no significant difference in the level of glucose between treatments (p>0.05). The serum lysozyme (LZM) activities, Complement system, C3 and C4 had a significant effect on different treatments (p<0.05). The highest and lowest activity of lysozyme, total Complement system (ACH50), and the level of C4 and C3 proteins, were observed in treatment /1 F diet and control, respectively. Antibacterial activity of serum was significantly different in control group with experimental groups (p<0.05). The highest and lowest levels of antibacterial serum were observed in 0.1 F diet and 2, respectively. According to the results of this study, it can be concluded that fucoidan stimulates the innate immune system of rainbow trout.