Background and Objective: Postpartum hemorrhage (PPH) is one the most frequent causes of maternal morbidity and morbidity, particularly after cesarean section. Oxytocin is a very helpful medication in this regard; however, its optimal dose and route of administration is still under debate. This study aimed to compare the effect of different doses of oxytocin in preventing PPH due to atony. Methods and Materials: In this randomized clinical trial, a total of 400 pregnant candidates for cesarean section were randomly received either 20 IU or 60 IU oxytocin diluted in 500 mL of serum. Pre- and post-cesarean levels of serum hemoglobulin, hematocrit, and blood pressure, as well as post-cesarean need of extra medication, incidence of atony and blood transfusion requirement were compared between the two groups. Results: Decrease in serum hemoglobulin, hematocrit, systolic blood pressure and diastolic blood pressure, as well as increase in pulse rate after cesarean section was significantly more dramatic in those received lower dose of oxytocin. The need of extra medication, atony and blood transfusion requirement were observed in 18%, 5.5%, and 3.5% of those treated with low dose oxytocin. There were absent in patients treated with high-dose oxytocin (p<0.05). Discussion and Conclusion: Compared to low-dose oxytocin, high-dose oxytocin is more effective in preventing atony and post-cesarean bleeding with no major side-effects.