In this study, the effects of different concentrations of ) Arthrospira platensis ( on the growth and antioxidant defense mechanism of koi fish (Cyprinus carpio (were investigated during a 8 weeks culture period. ). An experiment was designed with five treatments, including diets with 0, 2.5, 5, 7.5 and 10% of Arthrospira platensis and each with three replicates. One hundred and fifty koi fish with average initial weight of 30±1g were assigned to fifteen experimental tanks.The results revealed that the Superoxide dismutase(SOD) and catalase(CAT) activity level  in 10% (Arthrospira platensis) treatments demonstrated a significant increase compared to the control group (P<0.05)   and in all samples taken at different  treatments of the experiment did not show any significant differences(P>0.05). Liver enzymeze also decreased significantly by dietary supplementation (p<0.05). Also Koi fish  fed with 2.5% A. platensis showed a significant lower growth performance than those fed with 7.5 and 10% A. platensis (p<0.05). These results revealed that, inclusion of 10% A. platensis in diets did appear to have a significant positive effect on antioxidant defense mechanism , liver enzymes activities and growth rates of koi fish.