Human peritoneal mesothelial cells (HPMCs) in intact mesothelium have been demonstrated to protect against tumor peritoneal metastasis. We have previously reported that gastric cancer cells can induce peritoneal apoptosis, lead to damage of peritoneum integrity, and therefore promote peritoneal metastasis. In this study, we investigated the effects of TGF-beta1 on tumor-mesothelial interaction. Briefly, the levels of various soluble factors, in particular TGF-beta1, were measured. HMrSV5 cells, a human peritoneal mesothelial cell line, were co-incubated with TGF-beta1, gastric cancer cells, or gastric cancer cells and TGF-beta1 receptor inhibitor SB431542. The expressions of smad 2/3 and phosphorylated smad 2/3, indicator of TGF-beta/Smads pathway activation, were evaluated. Then the morphological changes of HPMCs were observed. The cell damage was quantitatively determined by fluorescent microscopy and flow cytometry. Tumor-mesothelial cell adhesion was also examined. Results showed a significant elevation of TGF-beta1 expression, which is companied by dramatically increased phosphorylated-smad 2/3 levels, after mesothelial cell co-culture with the gastric cancer cell line. In addition, mesothelial cells exposed to gastric cancer cells or TGF-beta1 became exfoliated and exhibited signs of injury, while blocking TGF-beta1 can partially inhibit these effects. These results indicate that soluble factors, such as TGF-beta1, produced in autocrine/paracrine manner in the peritoneal cavity, affect the morphology and function of mesothelial cells so that the resulting environment becomes favorable for peritoneal metastases.