We report a case of extrahepatic portal vein aneurysm (PVA) in the late postoperative period after orthotopic liver transplantation (OLT). A 46-year-old man underwent OLT in 2001 for treatment of hepatitis B virus-induced cirrhosis and received ultrasound follow-up for 9 years. The long-term ultrasound observations showed that the diameter of the main portal vein had progressively increased from 12 mm in 2001 to 34 mm in 2007, and then remained at 34 mm from 2007 to 2010. The patient did not show any symptoms or complications that required surgical treatment during the clinical follow-up. Our case suggests that PVA after OLT can remain without a sign of deterioration for a long time, and careful observation by ultrasound may be one of the options in the clinical management.