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Background & Aim: Preterm labor is the predominant cause of prenatal mortality and morbidity. The aim of this study was to evaluate the relative efficacy of indomethacin versus magnesium sulfate in the management of preterm labor. Methods: Sixty patients with preterm labor were randomly assigned into two groups. The first group received rectal suppository indomethacin and the second group received intravenous injection of magnesium sulfate (7 patients missed during the study). The results of delaying the labor, up to 72 hours and more than 72 hours, were compared in two groups. The collected data were analyzed, using chi-square. Results: In the indomethacin group , gestation was prolonged by 72 hours in 10 (37%) of women, and more than 72 hours in 17 women, but in the second group which received magnesium sulfate, 20 women had labor in the first 72 hour and only six women after 72 hours. The differences between the two groups were statistically significant (p<0.05). Conclusion: Results of this study confirmed an obvious difference in delaying labor in indomethacin and magnesium sulfate receiving groups. Indomethacin is cost effective and its administration is non-invasive. Therefore its use can be recommended in management of preterm labor.