The current research aimed to examine the effects of nano and macro iron oxide (Fe2O3) on growth performance, activity of hepatic enzymes, serum biochemical parameters, liver histopathology and appetite-related gene transcript in goldfish (Carassius auratus). Juvenile goldfish (4.3 g) were fed purified diets based on casein as a protein source and containing different levels of supplementary nano (0, 0.2, 0.5 and 1 g nFe2O3/kg diet) and macro iron oxide (0, 0.2, 0.5 and 1 g Fe2O3/kg diet) for 60 days. At the end of the experiments, blood, intestine and liver samples were obtained for histopathology, biochemical analysis and assays of ghrelin and IGF-I gene expression. The results showed that fish fed a diet supplemented with 0.5 g/kg Fe2O3 and nFe2O3 had a significantly (P < .05) greater specific growth rates (SGR) and lower feed conversion ratios (FCR) than those fed diets of 0, 0.2 and 1 g/kg Fe2O3 and nFe2O3. Furthermore, dietary Fe2O3 and nFe2O3 supplementation were found to increase plasma concentrations of alkaline phosphatase (ALP) compared with the control group (P < .05) while significant differences were not found in glucose, triglyceride and total protein (P > .05). Histopathological analyses exhibited some liver lesions, including blood cell aggregation, Lipidosis, hyperemia in goldfish. The results revealed that administration of Fe2O3 significantly (P < .05) up-regulated IGF-1and ghrelin gene expressions; these were significantly (P < .05) higher in Fe2O3-fed fish at the end of the trial. In brief, our findings indicate that both nFe2O3 and Fe2O3 forms of iron have a positive effect on general physiological parameters of the fish up to supplemental level of 0.5 g/kg. Also our results showed that Fe2O3 and nFe2O3 stimulate ghrelin and insulin-like growth factor-1 secretion from the intestine and liver of goldfish.