We report a case of non-cirrhotic portal hypertension in a 73 year-old woman, who had 19-year history of idiopathic myelofibrosis. There were esophageal varix, splenomegaly, and ascites. The biopsied liver showed irregular sinusoidal/ perisinusoidal fibrosis and occasional central-to-central fibrous connection. In areas with extensive fibrosis, coarse collagen fibers filled the sinusoidal spaces and compressed hepatocytes. However, nodular regeneration was absent. Double immunohistochemical stain for smooth muscle actin and proliferation cell nuclear antigen (PCNA) revealed diffusely activated stellate cells, some of which showed nuclear PCNA staining. There was also extramedullary hematopoiesis with bizarre megakaryocytes. The portal vein and its branches were patent. Idiopathic myelofibrosis is a rare cause of non-cirrhotic portal hypertension: the portal hypertension was considered to be the result of sinusoidal/perisinusoidal fibrosis in this case.