The present study was conducted to determine the possible use of supplemental oxygen treatment during the last 3 days of incubation on broiler performance, ascites susceptibility and some physiological traits. On the 18th day of incubation, fertile eggs were divided into two groups. From 18 to 21 d, the eggs were placed in two hatcher cabinets: one being a control at normal atmospheric conditions, 20.9 +/- 0.5% at 754 +/- 2 mm Hg and the other supplemented with oxygen at 23.0 +/- 0.5%. Oxygen supplementation had a significant effect on initial chick weight. At 6 weeks of age, body weight, growth rate, and feed consumption increased as partial pressure of O-2 rose from 20.9 to 23.0%. Feed conversion ratio decreased with the increasing partial pressure of oxygen. In the present study, general mortality and mortality due to ascites did not differ between the groups. At 5 weeks of age, oxygen supplementation did not cause significant changes in RBC (red blood cell count), Hb (hemoglobin) and Glucose. However, PCV (hematocrit) increased significantly as the partial pressure O-2 rose from 20.9 to 23.0%. There were no differences in the right ventricle, left ventricle + septum, total ventricle and RV: TV (right ventricle: total ventricle) ratio between the oxygen supplemented and control group.