|Preterm labor is the leading cause of prenatal and neonatal mortality morbidity and long term neurodevelopmental problems so that different treatments have been employed in order to suppress preterm labor from several years ago. Magnesium sulfate is often used as first line in suppressing of preterm labor. Side effect of this: thirst, hyperthermia, headache, diplopia, respiratory depression and rare cases respiratory paralysis and arrest. In later pregnancy progesterone may be important in maintaining uterine quiescence by limiting the production of stimulatory prostaglandins and inhibiting the expression of contraction associated protein genes with in the myometrium natural progesterone administrated vaginally is considered is effective in suppression of preterm birth and safe for both mother and fetus. We take a decision that compares the ability of magnesium sulfate with progesterone in suppression of preterm labor. Premature labor, which occurs in 7.23% of deliveries, is associated with prenatal morbidity and mortality American Academy of Pediatrics and American College of Obstetricians and Gynecologists have suggested below criteria for diagnosis of preterm labor: Presence of 4 uterine contractions in 20 minutes or 8 contractions in one hour accompanied by progressive cervical changes, Cervical dilatation greater than one centimeter, Cervical effacement equal or greater than 80%.