Background: Prostaglandins are effective in the ripening of the cervix and facilitating labor induction. Vaginal pH is probably an important factor in the effectiveness of vaginal prostaglandins. The aim of this study was to evaluate the effect of vaginal pH on the function of prostaglandin vaginal tablet during labor. Methods: This is a double-blinded clinical trial study of 147 pregnant primigravid trated in the Yahyanejad Hospital of Babol (Iran) from January 2006 to December 2007. Initial pH was measured during vaginal examination with nitrazin paper and the Bishop score was determined. All women received vaginal dinoprostone inserted in the posterior fornix of the vagina for cervical ripening and the second dose was administered if the uterine contractions were inadequate. Reassessment of the Bishop score after 12 hours, duration of latent and active phases, and also the duration of the second stage of labor were compared between the two groups with low or high vaginal pH. Results: The incidence of Cesarean section was lower in women with high vaginal pH but was not statistically significant. The Bishop score after 12 hours, latent phase, and second stage durations were not different in the two groups of high or low vaginal pH, but active phase duration in patients with high pH was significantly shorter than those with low pH (p = 0.019). Conclusion: High vaginal pH influences the function of prostaglandin tablet as a reduction in duration of the active phase of labor.