Background: Most patients have peritoneal metastasis over the course of gastric cancer. The prognosis of these patients with peritoneal metastasis is extremely poor. The aim of this study is to elevate the clinic significant of the expression of Carcinoembryonic Antigen mRNA (CEA mRNA) in peritoneal lavage in stage III gastric cancer. Methods: From 2013 to 2016, RT PCR were used to test CEA mRNA in peritoneal lavage in 100 patients after surgery with pathological stage III gastric cancer in Drum Tower Hospital, Medical School of Nanjing University and The Affiliated Jiangning hospital with Nanjing Medical School. According to the result of CEA mRNA in peritoneal lavage, patients were divided into CEA-positive group and CEA-negative group. All the patients received postoperative adjuvant chemotherapy, 1-year disease-free survival rate was compared between the two groups of patients. Results: Of 100 patients, 45 were positive for peritoneal lavage CEA mRNA. The higher positive CEA mRNA rate was associated with higher T stage, or higher N stage. The 1-year diease-free survival rate for patients with negative CEA mRNA was statistically significant higher than that for patients with positive CEA mRNA (84% vs 57%, p < 0.05). Conclusions: Peritoneal lavage CEA mRNA can predict the prognosis of patients with stage III gastric cancer. Patients with positive CEA mRNA in peritoneal lavage should consider a more positive treatment including intraperitoneal therapy.