Objective: To compare the efficacy and safety of 100 μg oral misoprostol for induction of labor between the regimen of 3 hour and 6 hour interval administration. Methods: Singleton pregnancies indicated for induction of labor between 34 and 42 weeks of gestation in the condition of unfavorable cervix (Bishop score ≤4) and no contraindication for prostaglandins therapy were recruited into the study. All pregnant women were randomly assigned to receive 100 μg oral misoprostol every 3 hours or 6 hours until the cervix was favorable for amniotomy, spontaneous rupture of membranes or active labor occurred. Results: The mean time interval from induction to vaginal delivery was significantly shorter in the 3 hour interval group, compared with the 6 hour interval group (13.82±6.98 h and 17.66±7.48 h, P = 0.0019). There was no significant difference between the groups with regard to mode of delivery, analgesic requirement, maternal complication and neonatal outcome. Conclusion: 100 μg oral misoprostol every 3 hours is more effective for labor induction than every 6 hours but there was no difference in mode of delivery, analgesic requirement, maternal complications and neonatal outcome. A dose of 100 μg misoprostol orally every 3 hours seems to be the optimum regimen and the new option for labor induction. However, further study should be performed.