Abstract: Objective To study the efficiency and safety of Cook cervical expansion balloon in promoting cervical ripening at third trimester of pregnancy. Methods One hundred primiparae at third trimester of pregnancy with labor induction indications and their cervical score < 4,admitted to our hospital from August 2009 to August 2010,were enrolled in this study and divided into study group(n=42) and control group(n=58).Primiparae in study group selected Cook cervical expansion balloon for induction of labor while those in control group selected oxytocin for induction of labor.Cervical ripening,labor induction and infection risk of the two methods were compared between the two groups. Results The cervical ripening efficiency and success induction rate of labor were significantly higher in study group than in control group(97.6% vs 36.2%,73.8% vs 37.9%,P<0.01).However,no significant difference was found in temperature and WBC between the two group(P>0.05). Conclusion Cook cervical expansion balloon can effectively expand the uterine neck,promote the cervical ripening in poor condition and does not increase the infection rate during delivery.