A total number of Two hundred and forty , day old unsexed Pekin and Domyati ducklings (120/each breeds) at one-day old were used in the present study. The experimental period extended for 12 weeks of age. Experimental ducks were randomly divided into four equal treatments (30 Pekin or Domyati ducks each) in three equal replicates (10 ducks each). Ducks were fed diets containing Potassium iodide at levels of 0, 1, 1.5 and 2 mg iodine/kg ration for T1, T2, T3 and T4, respectively. The results showed that growing ducklings fed ration supplemented with Potassium iodide (2, 1.5 and1%) were significantly increased live body weight, body weight gain, feed intake, feed conversion ratio, shank and keel length and body circumference. Relative weight of carcass, and lymphoid organs were significantly increased with iodine. Moreover, blood hemoglobin, red and white blood cells counts and lymphocytes cells% were significantly increased by iodine supplementation to diets. On the other hand, heterophils (%) and H/L ratio were significantly lower by different levels of Potassium iodide supplementation. As well as, plasma total protein, albumin, globulin, A/G ratio, calcium, phosphorus and T 3 were significantly higher in treated than the control groups. Conversely, duckling fed ration supplemented with Potassium iodide recorded a significant decrease in plasma total lipids, cholesterol, triglyceride, AST and ALT concentrations .Conclusively, potassium iodide at 2mg/kg diet level resulted in better performance without any adverse effect on physiological responses and carcass quality traits .