Objective To evaluate efficacy and safety of ursodeoxycholic acid(UDCA)for preven-tion of obstruction of biliary plastic stent.Methods A total of 88 patients with plastic biliary stents at our hospital were divided into UDCA group and the control group.Stents were collected 3 months after the opera-tion for argile biliare in the stent.The contents of APF,calcium bilirubin and calcium carbonate were ana-lyzed by biochemistry,and bacterium culture was conducted.Results The obstruction rate of UDCA group (n=43)and control group(n=45)were 8. 9% and 58. 1% respectively with significant differences(P<0. 05).There were significant differences in the protein level of APF between the UDCA group(17. 29 ± 9. 52)μm/L and the control group(10. 39 ±2. 17)μm/L(P<0. 05).Calcium bilirubin[(13. 90 ±3. 80) vs.(30. 92 ±7. 07)]and calcium carbonate[(12. 60 ±5. 69)vs.(16. 52 ±4. 11)]were less than those of the control group (P<0. 05 ).Only one type of bacterium was found,and Escherichia coli were cultured in 40. 0% samples,a lower incidence compared with the control(72. 1%).Conclusion UDCA would lessen biofilm and smooth stent surface by promoting secretion of endogenous bile acid and increasing APF,and keep the biliary patency.