In advancing gastric cancer, especially when the serous is invaded, the plantation of cancer cells in peritoneal is common and it affects patients' survival time severely. Based on successfully labeled McAb (monoclonal antibody) 3H11 with ¹⁸⁸Re, we investigated the effect of RIT (Radioimmuno-Therapy) with ¹⁸⁸Re-3Hll on preventing the peritoneal micrometastasis of gastric cancer cells in nude mice to increase the survival time. After 1×10⁶ BGC-823 gastric cancer cells were injected into the peritoneal cavity of each mouse, 45BABL/C nude mice were divided into 9 groups. Each group received different doses of ¹⁸⁸Re-3Hll or ¹⁸⁸Re-IgG, or saline I.P.16 hours postoperation. The injected volume of each mouse was 1.0mL. The results showed that the survival time depended on the injected doses during 0 to 37 MBq. The survival time was 170±25.3d after 37 MBq ¹⁸⁸Re-3H11 were treated. It was over 5 times more than that for the saline group and about 3 times more than that for 37 MBq ¹⁸⁸-Re IgG group (p<0.05). The mice hemogram were reduced to lowest 14 days after injection, but they recovered after 28 d. Conclusion: with proper injection doses,early postoperative ¹⁸⁸Re-3H11 I.P. is effective and safe for the prevention of intra-peritoneally injected gastric cancer cells from surviving,growing and disseminating in nude mice.