The objective of the present study was to compare the effect and complications of (oxytocin antagonist) and nifedipine(Ca channel blocker) for treatment of preterm labor. A randomized clinical trial study was performed on 80 pregnant women with preterm labor between 26-34 weeks of pregnancy in Shahid Akbar Abadi Hospital in Tehran. 40 women(atosiban group) were compared with 40 women(nifedipine group) for drugs’ safety and efficacy(delay of delivery for more than 48 hours for steroid therapy). Duration between drug administration and delivery was also compared. There was not any significant difference between two groups in treatment of preterm labor. Atosiban was effective in 82.5% of cases and nifedipine in 75%. Side effect in atosiban group was 17.5% and in nifedipine group 40%, which was a statistically significant difference(P=0.027). Duration between treatment and delivery was 29.03 ± 16.12 days in atosiban group and 22.85 ±13.9 days in nifedipine group without statistically significant difference. In conclusion, atosiban is an effective and safe drug for treatment of preterm labor with minimal side effects and can be an option in treatment of preterm labor specially in patients with heart disease and multifetal pregnancy.