Background: To enhance the spread of spinal anesthesia, fentanyl may be administered intravenously (IV). This prospective study aims to investigate the effect of fentanyl administered IV on the spread of spinal anesthesia and assesses side effects of intravenous fentanyl. Methods : Forty patients were ran- domly assigned into two groups receiving either spinal anesthesia with isobaric bupivacaine 8 mg or isobaric bupivacaine 8 mg plus fentanyl intravenous. The level of the first peak of sensory block and motor block (modified Bromage scale) were recorded. Twenty minutes later, control group was administered normal saline 0.02 ml/kg intravenous whereas fentanyl group was administered fentanyl 1 microgram/kg intravenous. The level of second peak sensory block, motor block, side effect and duration of anesthesia were recorded.  Results : The first peak dermatomal level sensory block (cold and pinprick test) was T6 (T2-T12) and T9 (T3-L3) for control group compared with T8 (T3-L3) and T10 (T3-L3) for fentanyl group. After study drug administration, number of patients with sensory block enhancement in fentanyl group were 17 and 19 patients by cold and pinprick test significantly more than control group which were 8 and 7 patients respectively ((P < 0.05). All patients have completely motor block. The incidence of hypotension, bradycardia and duration in PACU was not different in both groups. Conclusion : The present study demonstrated that intravenous fentanyl can enhance the spread of spinal anesthesia whereas it does not increase the incidence of respiratory depression, sedation, nausea and vomiting, urinary retention, hypotension, bradycardia and duration in PACU.