BACKGROUND Preterm labor is one of the most important causes of hospitalization during pregnancy and can lead to serious complications in neonates. OBJECTIVE This study aims to compare the effect of transdermal nitroglycerin (TNG) patches and sublingual tablets of Isosorbide dinitrate (ISD) for the prevention of preterm delivery. METHODS A total of 110 healthy pregnant women aged 18-35 years with a healthy and alive fetus and gestational age between 24-34 weeks who had at least 8 regular uterine contractions per hour were included in this single-blinded clinical trial. After exclusion, the women were randomly divided into TNG (n = 50) and ISD (n = 49) groups. After the first dose of medication (TNG or ISD), patients who developed complications such as hypotension, headache, or both, were also excluded from the study. RESULTS A total of 58 patients completed the treatment course (29 patients in each group). A significant difference in delayed preterm labor and recovery time was reported between the TNG and ISD groups. CONCLUSIONS Complications and the number of contractions were not statistically different in the two groups. We concluded that the TNG patch is more effective than ISD in delaying labor. Both drugs are likely to have a similar incidence of side effects.