Carnitine is mainly biosynthesized in the liver and kidneys, so carnitine deficiency has occurred when liver tissue is damaged by liver diseases such as cirrhosis and non-alcoholic fatty liver disease (NAFLD), although blood free carnitine is high in most cases. However, carnitine insufficiency is reported in liver diseases. Carnitine supplementation therapy to patients with clinical symptoms such as hepatic encephalopathy or severe painful muscle cramps in cirrhotic patients are very effective. Ammonia is a very toxic substance, and it is important to detect latent hyperammonemia by ammonia test and neuropsychological tests such as number-connecting test. Carnitine improves these abnormalities in neuropsychological tests, and it was also confirmed by near-infrared spectroscopy (NIRS) study. Also, carnitine administration suppresses sarcopenia and reduces hospitalization in cirrhotic patients. Carnitine administration showed beneficial results in several animal models of NAFLD.