Inferior mesenteric venous thrombosis (IMVT) is a very rare disease of colon ischemia. We experienced two cases of IMVT that required operations. The first patient was a 74-year-old male, who was admitted to our hospital because of melena and diarrhea. He was diagnosed with IMVT by angiography. As no improvement was seen after the conservative therapy for a month, left colectomy and transverse colostomy were performed. There was a small ulcer in the resected colon mucosa. The findings of histopathological examination revealed that mild and repeated ischemia of the colon had been caused. The second patient was a 70-year-old male, who was admitted to our hospital with the chief complaint of constipation, lower abdominal pain and nausea. He was diagnosed as IMVT by angiography. As no improvement was seen after the conservative therapy for a month, an operation was performed. The operative findings confirmed severe swelling of mesenteric fatty tissue and vascular ectasia of mesocolon. Left colectomy and transverse colostomy were performed. Histopathological examination of surgical specimens disclosed the multiple thrombi and almost complete occlusion of the inferior mesenteric vein, the invasion of lipid-filled macrophages as mesenteric panniculitis, and ischemic change in the sigmoid colon mucosa.