This study was aimed to use nano manganese oxide (Mn₂O₃) as a mineral premix on growth parameters followed by the erythrocyte and leukocyte properties of rainbow trout (Oncorhynchus mykiss) fingerlings. 450 specimens of rainbow trout fingerlings with an average weight of 9.1±0.3 g after acclimation to the experimental condition were randomly distributed in triplicate to five treatments of raceways ponds with open flow. Basal diet as control treatment was prepared by adding no supplemental manganese mineral. Other experimental diets were prepared by adding 4, 8, 12 mg and 12 mg nano manganese oxide followed by 12 mg manganese sulfate (as positive control treatment) to each kg diet. The Fish were fed for 8 weeks under experimental conditions with the corresponding diets. At the end of feeding trial, growth performance followed by the hematological parameters were determined by cutting the caudal peduncle (n=5). Results demonstrated no significant differences in final weigh, specific growth rate, protein efficiency ratio, and condition factor of the fish among the treatments. However, erythrocytes, hemoglobin, and hematocrit of fish fed by 12 mg kg^-1 nanoparticle manganese oxide were significantly higher than fish in other treatments (p<0.05), while no significant difference was found in erythrocyte indices including mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC) between the treatments. There is also no significant difference in former variables of the fish fed manganese sulfate and control diet. Findings of the present study showed that the use of nano manganese oxide increase some blood parameters of rainbow trout fingerlings including number of erythrocytes, hemoglobin, and hematocrit.