Objective To observe the expression of PECAM-1 in colorectal cancer in rat and to explore the mechanism of lymphatic metastasis of tumor. Methods Wistar rats were used to make the animal model of primary colorectal cancer, by using HE staining, the cancer type and progression of cancer were devided. By immunohistochemistry staining, the expression of PECAM-1 in tumor tissue,blood vessels and lymphatic vessels were observed. Results PECAM-1 expression in blood vessels and lymphatic vessels were observed in normal tissue, while its expression on tumor tissue and lymphatic vessels decreased according to the cancer progression. Conclusion PECAM-1 may be involved in the early process of tumor cells adhering to the endothelium; decreased expresson of PECAM-1 on lymphatics may be related with lymphangiogenesis and the opening of endothelial junction of lymphatic vessels