Abstract: Objective To investigate the value of serum cancer antigen 12- 5( CA12- 5) level in the diagnosis of peritoneal effusion in patients with liver cirrhosis. Methods A total of 142 patients with liver cirrhosis who were hospitalized at Beijing Tiantan hospital from January 2011 to December 2015 and met the inclusion criteria were enrolled. According to the presence or absence of peritoneal effusion,the patients were divided into peritoneal effusion group( 81 patients) and non- peritoneal effusion group( 61 patients). The two- independent-samples t test was used for comparison of normally distributed continuous data between groups,the Mann- Whitney U non- parametric test was used for comparison of non- normally distributed continuous data between groups,and the chi- square test was used for comparison of categorical data between groups. A Spearman correlation analysis was used to investigate the correlation between serum CA12- 5 level and the degree of peritoneal effusion. Results The peritoneal effusion group had a significantly higher serum CA12- 5 level than the non- peritoneal effusion group [290. 00( 50. 82- 618. 4) U / ml vs 15. 39( 9. 77- 23. 04) U / ml,Z =- 8. 537,P < 0. 01]. Serum CA12- 5 level was positively correlated with the degree of peritoneal effusion( r = 0. 812,P < 0. 01) and had a high diagnostic value for peritoneal effusion( the area under the receiver operating characteristic curve was 0. 92). Serum CA12- 5 level had a sensitivity of 81. 5% and a specificity of 88. 5%at a cut- off value of 35. 00 U / ml. Conclusion Serum CA12- 5 level can help with the diagnosis and monitoring of peritoneal effusion.