Extensive use of zinc oxide nanoparticles (ZnO-NPs) has raised concerns about the environmental risks caused by these NPs entering into aquatic ecosystems. The aims of the present study were to assess the short term effects of ZnO-NPs on activities of metabolic enzymes and haematological parameters of the Caspian trout juveniles. Fish were exposed to 10% of LC_50-96h for 4 days. Blood samples were collected from fish after 24, 48, 72 and 96 hours of exposure. Analysis of blood parameters showed that the numbers of red blood cells, haematocrit, haemoglobin, white blood cells, neutrophils and monocytes were significantly increased as compared to those of the control group (P<0.05), whereas the numbers of lymphocytes were decreased significantly compared to that of the control group (P<0.05). Enzyme activity levels of ALP, AST and ALT after 96 hours of exposure and enzyme activity level of LDH after 48 hours of exposure to ZnO-NPs were significantly increased as compared to those of the control group (P<0.05). It can be concluded that the sensitivity of the Caspian trout juveniles to the sub-lethal concentration of ZnO nanoparticles were relatively high. Therefore, alteration in haematological parameters can be a useful indicator for studying the short term effects of ZnO-NPs.