Background: in the developed world, preterm birth (PTB) is a major problem in modern obstetrics; its prevalence is still rising in many industrialized countries. Early PTB (before 34 weeks) is particularly associated with high rates of mortality and morbidity. Aim of the Work: to compare the efficacy of oral nifedipine, intra venous ritodrine and magnesium sulfate as tocolytics in cases with preterm labor and evaluate their effects on the utero placental perfusion in order to choose the safest and most cost-effective drug. Patients and Methods: a prospective study that was conducted in El sayed Galal and El Hussien University hospitals from November 2016 to November 2018. By identifying 150 pregnant women with gestational age from 24-37 weeks gestation with preterm labour pain, intact membranes, singleton pregnancy. Results: a statistically significant difference was found between groups according to fetal umbilical artery PI before and after treatment. Also, significant difference between before and after treatment according to fetal umbilical artery PI in magnesium sulfate group. Also a statistically significant difference between groups according to fetal middle cerebral artery PI before and after treatment. Also, significant difference between before and after treatment according to fetal middle cerebral artery PI in nifedipine and magnesium sulfate group. Conclusion: there was no overall difference between nifedipine, magnesium sulfate and ritodrine, in their efficacy as tocolytic for preterm labor. However, Nifedipine had fewer maternal side effects followed by magnesium sulfate than ritodrine. Irrespective of their tocolytic effects, magnesium sulfate has the most significant effect on Doppler study.