Abstract: Objective: To study the effect of arsenic trioxide (As₂O₃) on human colon carcinoma cell line implanted in nude mice in vivo, as well as the related mechanism and toxicity. Methods: The transplantable human colon carcinoma tumor models in nude mice were established and were divided into 4 groups at random. The anti-tumor effect of every group and general condition of the nude mice were studied. The specimen obtained from the mice were detected by optical microscope, transmission electron microscope (TEM), TdT-mediated dUTP nick end labeling (TUNEL) and immunohistochemical staining. Also mouse blood routine examinations were made. Results: As₂O₃ could obviously inhibit the growth of tumor, induce the apoptosis of carcinoma cell and regulate the related genetic expression, and As₂O₃ didn't induce hepatic, renal and hematopoietic system injury in the nude mice. Conclusions: The As₂O₃ injection has a significant anti-tumor effect on the transplant tumor models for human colon carcinoma in nude mice, which may be related to inducing the apoptosis of carcinoma cell and regulating several genes. As₂O₃ has no obvious injury for hepatic, renal and hematopoietic system of the nude mice.