Six hundred 1-d old male broilers (Ross 308) were assigned into four experimental groups; each was com-posed of 5replications of 30 birds including control (0), 10, 20 and 40 mg/kg nano-ZnO in the basal diets. Birds were exposed to low ambient temperature (15 to 18 oC) to induce ASCITES. Blood parameters including; activity of aspartate transferase (AST), alanine transferase (ALT), lactate dehydrogenase (LDH) and levels of protein, glucose, hemoglobin, hematocrit, white blood cell (WBC), red blood cell (RBC) and malondialdehyde (MDA) were determined. Mortalities were inspected to determine cause of death and di-agnose of ASCITES. At the end of the experiment (day 42), 2 chickens from each replicate were randomly selected, slaughtered and ASCITES index calculated. Average body weight gain (ABWG) and average feed intake (AFI) were measured weekly and weekly average feed conversion ratio (AFCR) was calculated. Re-sults showed that 40 mg/kg nano-ZnO significantly reduced MDA level in plasma and liver. Moreover, 20 and 40 mg/kg nano-ZnO had decreased significantly mortality due to ASCITES and ASCITES index. It is also, nano-ZnO in all levels had significantly increased ABWG. Birds in the 40 mg/kg nano-ZnO group had lower AFCR. It was concluded that nano-ZnO improved performance and reduced mortality due to ASCITES, and 40 mg/kg nano-ZnO is the optimal level in diets.