The present study was to scrutinize the protective effect of dietary cinnamon essential oil supplementation on reducing the toxicity of aflatoxin B₁ to rainbow trout fingerlings regarding hematological indices, serum biochemistry and liver histopathology. To that end, fish were randomly allotted into 6 distinct treatments composed of different combinations of three dietary aflatoxin (0, 25 and 50 ppb) and two dietary cinnamon essential oil (0 and 1 percent) inclusion levels with three respective replicates. The experiment lasted for 60 days. At the end of the experiment random samples were taken to study the aforementioned indices. Hematocrit, red blood cell count and hemoglobin concentration were positively affected by dietary supplementation of cinnamon essential oil (P<0.05). Dietary aflatoxin also resulted in increased red blood cell count and hemoglobin concentration (P<0.05). Activities of ALP and ALT were increased due to dietary aflatoxin contamination (P<0.05); however, supplementation of cinnamon essential oil lowered the activity of ALP in serum of aflatoxin exposed fish (P<0.05). Total protein and globulin content were only affected by aflatoxin and cinnamon essential oil (P<0.05), while, the interactive effect of aflatoxin and cinnamon essential oil significantly affected albumin and lysozyme activity of serum (P<0.05). Hepatosomatic index of fish were significantly decreased by dietary aflatoxin exposure (P<0.05). Histological observations also showed blood vessels dilation, cytoplasmic degeneration, blood congestion, immune cells infiltration and necrosis in a dose dependent manner. In conclusion, 1 % dietary supplementation of cinnamon essential oil resulted in decreased deleterious hepatic tissue changes and also improvement of hematological and serum biochemical indices of fish fed aflatoxin contaminated diet especially of those fish exposed to 25 ppb dietary aflatoxin.