In this study the effect of black cumin seed (Nigella sativa) on nonspecific defense mechanisms of rainbow trout (Oncorhynchus mykiss) was investigated. The percentage of black cumin seed in diet were 0.0; 0.5; 1.0; 2.5; 5.0; 10.0 and 20.0 g/kg respectively. Rainbow trout (O. mykiss) of initial weights 108.7±17g were used for experiment. The biometric measurements and the blood parameters were determined in each group after feeding 7, 15, 25, 40 and 60 days respectively. In 7 day experiment, plasma protein, MCH, MCHC, RDW_SD and RDW_CV, MPV values at 1.0 and 2.5 g/kg dose groups differed favorably from the control group. In 15 day feeding experiment, there is a positive mathematical difference in almost all parameters in the 2.5 g/kg dose group compared to control group. In 25 day experiment, there is a positive mathematical difference according to control group in almost all parameters and statistically significant difference between MCH and MPV values in the 2.5 g/kg dose group (P<0.05). In 40 day feeding experiment, plasma protein, MCV, RDW_CV, PLT values differed positively from the control group at the 1.0 and 2.5 g/kg dose groups and mathematical differences were observed in other parameters. In 60 day feeding experiment, at the doses of 1.0 and 2.5 g/kg, many parameters differ from the control group and from the other groups.