Objectives: Opioids are most widely used for pain relief during childbirth. An alternative opioid, fentanyl, has been shown to be a good option for pain management and has fewer side-effects on both mother and fetus. Therefore, the purpose of this study is to determine the effect of fentanyl on pain as well as the duration of the active phase of labor. Methods: This clinical trial was conducted on 70 multiparous parturients having labor from May to July 2006 at Tamin Ejtemai Hospital. They were selected by convenience sampling at the beginning of the active phase of labor. The samples were then randomly divided into the case and control groups. The case group received fentanyl 50 micrograms in two doses, one hour apart after being diluted (0 and 60 mins). Vital signs were recorded pre-administration and 5, 15, 30, 45, 60 minutes post-administration. Pain intensity was estimated by visual pain scale (0-10) four times (before and 1, 2, 3 hours after the intervention). Data analysis was done using the student t-test, repeated measure ANOVA and chi-square tests via SPSS 11.5 software. Results: The results showed a significant reduction in pain (p=0.002) and HR (p=0.001) in the case group. The mean pain score also decreased from 8±1 to 5±1. There was a significant difference in terms of the duration of the active phase between the two groups (p=0.001). However, there were no significant differences in terms of systolic blood pressure and diastolic blood pressure between the two groups. Conclusion: Fentanyl provides good analgesic effect for pain management during labor by considerably reducing the duration of the active phase, and can therefore be used as an acceptable analgesic agent during labor.