Background: Considering high complications of labor induction with drugs, finding an easy, inexpensive, confident method for induction of labor is important. This study was performed to evaluate the effect of castor oil on initiation of labor in pregnancies after 40 weeks in Shabihkhani Hospital in Kashan during 2004-2006. Materials and Methods: This randomized clinical trial study was performed on 200 women after their 40 weeks of pregnancy. Women were divided to two groups (100 women in case group and 100 women in control group). The case group took 60 ml castor oil while control group didn’t take any treatment. The two groups were compared for onset of labor in 24 hours, time of onset of labor, delivery time and method and Apgar score of their newborns by X2, CI, OR and t-tests. Results: Labor was induced in 70% of women in the case group versus 12% in the control group during 24 hours (p<0.001, OR=17.11, CI=8.17-35.84) and vaginal delivery happened in 28% of women in the case and 1% of women in the control group (p<0.001. OR=29.5. CI=5.86-226.4). There was no significant difference between the two groups in the method of delivery and newborns Apgar score. Conclusion: The use of castor oil can induce labor in pregnant women after 40 weeks without any increase in the complications of pregnancy including caesarean section, and drop in Apgar score. To confirm findings, it is recommended to perform further clinical randomized studies.