Background: Magnesium sulfate has been used in different studies as a pain control agent; however, the results are contradictory. This study aimed to evaluate the effect of magnesium sulfate on postoperative pain management during abdominal surgery. Methods: This randomized double-blinded study involved 84 patients candidates for abdominal surgery, who were divided into two groups (42 patients in each group). In the magnesium group, 25 mg/kg magnesium sulfate was infused for 1 h; then, 100 mg/kg/24 h was infused in the intensive care unit. The patients’ pain intensity, which was the primary outcome, was assessed using the numeric rating scale (NRS) every 3 h. If the NRS was > 3, morphine was used and evaluated as a secondary outcome after 24 h. The patients and nurses who assessed the patients were blinded to the drug groups. The results were analyzed using SPSS ver. 19 software, and statistical significance was set at P < 0.05.Results: Age, sex, body mass index (BMI) and surgery duration were similar between the groups. The pain intensity was 4 ± 0.2 in the control and 3.8 ± 0.2 in the magnesium group at first and then 3.98 ± 0.2 in the control and 3.45 ± 0.2 in the magnesium group at the third hour, which were similar in both groups (P = 0.39 and P = 0.17, respectively), but thereafter between 6 and 24 h, the pain severity was significantly lower in the magnesium group (4.4 ± 1.3 in the control and 3.34 ± 1 in the magnesium group at 6th hour and P = 0.001). In addition, morphine intake in the first 24 h in the two groups had a significant difference, with 13.2 ± 5.7 in control group and 8 ± 3.5 in magnesium group (P = 0.001). Hemodynamic state and muscle relaxation were similar in both groups.Conclusions: In this study, intravenous magnesium sulfate after abdominal surgeries for 24 h resolved the pain intensity after six hours and reduced morphine dosage.