Using of natural plants as immunostimulant in fish is more useful than antibacterial drugs that cause adversely side effects for fish, environment and consumers. Therefore, we investigated the effect of black cumin seeds, Nigella sativa, as a herbal medicine plant on some immuno-hematological parameters and specific as well as non-specific defence mechanisms of rainbow trout, Oncorhynchus mykiss. Fish were fed with basal diet incorporated with 1, 2.5 and 5% Nigella sativa for 21 days. At the end of experiment, the hematocrit (Hct), leukocyte levels, glass-adherent NBT positive cell activation, serum protein and total immunoglobulin level were determined in fish blood. No significant differences (P >0.05) were found in hematocrit levels of fish fed the diets containing 1 and 2.5% black cumin seeds, but it was significantly (P <0.05) increased in fish receiving diet containing 5% black cumin seeds compared to the control. Glass-adherent NBT positive cell activation and leukocyte level in black cumin supplemented groups showed no significant differences (P >0.05) when compared to the control group. The serum protein and total immunoglobulin levels were significantly (P <0.05) higher than those of the control group. In conclusion, based on the current findings together with the low cost and the immunostimulative effect of black cumin seed, it is recommended to be used in fish feed to minimize the mortalities caused by some pathogens.