Abstract: Objective: To detect the expression of DDC mRNA in the peritoneal washings of gastric cancer and to detect investigate its relationship with peritoneal metastasis of gastric cancer. Methods: The expression of DDC mRNA in peritoneal washings of 92 patients with gastric cancer and 10 patients with benign lesions was detected by Trizol and RTPCR. Peritoneal lavage cytology (PLC) examination was performed conventionally. Ten samples of peritoneal washings of benign lesions were used as the control. Results: The expression of DDC mRNA in peritoneal washings was found in 57 cases with gastric cancer, and the positive rate was 62.0%, closely related to the PS factors. The level of DDC mRNA expression was closely related to invasive depth, serosal type, PLC examination and existence of gross peritoneal metastatic focus (P<0.05). No DDC mRNA expression was found in the 10 samples of benign peritoneal washings. Conclusion: The expression of DDC mRNA in peritoneal washings of gastric cancer is closely related to the peritoneal metastasis, and it is a sensitive index to predict subclinical metastasis to the peritoneum in patients with gastric cancer.