Nasal polyps are associated with the inflammation of the nasal cavity and the sinus mucosa. When medical treatment cannot solve a patient’s problem, a functional endoscopic sinus surgery may be indicated. Bleeding impairs the surgery field during operation and increases the operation risk and time. Pre-operative corticosteroids can reduce bleeding during surgery. In this study, we have evaluated the effect of pre-operative single-dose prednisolone (1 mg/Kg/dose 24 h before surgery) versus 5-day prednisolone (1 mg/Kg/day before operation) on the bleeding volume and the surgery field quality during FESS. In this mono blind randomized clinical trial, 80 patients with bilateral nasal polyps were randomly assigned in two groups. The first group (A) received a single dose of 1 mg/Kg/dose prednisolone on the day before the surgery. The second group (B) received 1 mg/Kg/day prednisolone for 5 days before the operation. The patients were operated on under general anesthesia through the same protocol. The mean arterial blood pressure was 70–80 mm Hg in both groups. The surgeons were not aware of the patients’ group. The bleeding volume and the surgeons’ opinion about the surgery field quality were recorded at the end of the procedure and analyzed by Chi-square and t test. The two groups were not significantly different in their overall demographic and clinical characteristics. The mean bleeding volume during the operation was 266.5 ± 96.31 ml in group A and 206 ± 52.81 ml in group B; there was a significant difference between the groups (P value = 0.038). There was no significant difference between the groups in the surgeons’ opinion about the surgery field quality (P value = 0.09). In conclusion, unlike a single dose (1 mg/kg/dose), treatment with 5-day prednisolone (1 mg/kg/day) can reduce blood loss during FESS more efficiently and may improve the surgery field quality slightly. But this difference is not clinically significant.