In colorectal cancer, progression with an intravenous tumor thrombus is very rare. Here, we report 2 cases of colorectal cancer which showed a tumor thrombus in the inferior mesenteric vein (IMV). Case 1: A 69-year-old woman was admitted for the treatment of advanced rectal cancer, and underwent a low anterior resection. Six months of post-operative therapy was carried out with mFOLFOX6, but a metachronous lung metastasis was detected and a lung partial resection was performed. Case 2: A 67-year-old man was admitted for the treatment of advanced sigmoid colon cancer with simultaneous liver metastasis, and underwent a laparoscopic high anterior resection. Four courses of mFOLFOX6+bevacizumab chemotherapy were carried out after surgery, and subsequently he underwent a partial hepatectomy. In both cases IMV tumor thrombus was suspected from abdominal contrast-enhanced computed tomography (CT). Tumor thrombus filling the lumen of the IMV was confirmed on histopathological examination. Colorectal cancer with IMV tumor thrombus is a form of advanced cancer with advanced vascular invasion, and there is a high risk of simultaneous or metachronous hematogenous metastasis. Combined modality therapy should therefore be given to improve the prognosis.