This study was carried out to evaluate the effects of dietary essential oils of ginger (Zingiber officinale), black seed (Nigella sativa) and cone flower (Echinacea angustifolia) on the immune system of cultured rainbow trout, Oncorhynchus mykiss. A total of 360 fish were divided into 4 groups and 3 replicates each containing 30 fish. The fish were fed for 21 days with 1% of the herbal essential oils and with unsupplemented diet as the control group. Haematological and immune parameters including serum lysozyme, number of phagocytic bacteria, Hb, PCV, MCV, MCH, MCHC, number of RBC and WBC were studied. Treatments recorded enhancement in immune parameters compared to the control group. Significantly higher serum lysozyme level and phagocytic germs were detected in the groups fed black seed, ginger and cone flower (P<0.05). However, no significant difference was found in RBC, Hb, PCV, MCV, MCH and MCHC of fish fed the diets containing essential oils and the control group (P>0.05). The results of this study demonstrated that essential oils might strengthen the non-specific immunity of rainbow trout.