Objective To investigate the detection of peritoneal free cancer cells and its clinical significance. Methods The peritoneal free cancer cells, the positive rates of CK20 protein and CK20 mRNA expressions of peritoneal lavage fluid were detected by peritoneal lavage cytology (PLC), flow cytometry (FCM) and real-time fluorescent quantitative RT-PCR in 50 cases of gastric cancer patients, respectively. The sensitivity of three kinds of detection method to peritoneal free cancer cells was compared. Results The positive rates of peritoneal free cancer cells, CK20 protein and mRNA expression of peritoneal lavage fluid were 20.0% (10/50), 36.0% (18/50) and 58.0% (29/50), respectively. The positive rate of CK20 mRNA expression detected by real-time fluorescencequantitative RT-PCR in peritoneal lavage fluid was significantly higher than those of the CK20 protein expression detected by FCM and peritoneal free cancer cells detected by PLC (Plt;0.05 or Plt;0.001). The difference of positive rate of CK20 protein expression and peritoneal free cancer cells was not significant (Pgt;0.05). The positive rate of CK20 mRNA expression of peritoneal lavage fluid was related to the tumor invasion depth, differentiation degree, TNM stage, and lymph node metastasis (Plt;0.05). Conclusion Real-time fluorescence quantitative RT-PCR is an effective method for the detection of peritoneal free cancer cells.