This study was designed to evaluate the effect of Nannochloropsis oculata microalgae supplementation on 120 rainbow trout fingerling (initial weight of 2±0.3 g). Fish were randomly distributed into four groups and stocked into 12 aquaria (20 liter) in triplicates. Four experimental diets contained 0 (control), 7.5 (T₁), 15 (T₂), and 22.5 (T₃) g of the algae powder per kg of feed for 3 months. Lipid, liver enzymes, and some hematological properties of blood (i.e. WBC, RBC, HCT, MCV, HB, and MCHC) were measured at the end of the trial. Results showed that leucocyte population was not significantly different in all treatments (p>0.05). However, the highest erythrocyte counts and hematocrit index were obtained in T₂ (p<0.05). Also, the highest and lowest cholesterol content were recorded in control (279.3±1.47 mg/dl) and T₂ (173.3±1.47 mg/dl), respectively (p<0.05). Results indicated that supplemented diet with 15 g algae per kg of feed can significantly reduce the liver enzymes i.e. aspartate aminotransferase, alkaline phosphatase and alanine aminotransferase (p<0.05). The highest low-density lipoprotein was obtained in control group (68.00±0.75 mg/dl) and the lowest value was observed in T₂ (25.200±0.72 mg/dl) (p<0.05). Therefore, dietary supplementation of the fish diet with N. oculata at 15 g/kg of feed probably improved the blood indices and liver function.