Hyperammonemia is a severe metabolic disorder. If left untreated，it could lead to various degrees of damage of brain and liver，resulting in unconsciousness，psychomotor disturbance，brain edema，disability and high mortality. There may be several genetic and non-genetic diseases to cause hyperammonemia. The etiology of hyperammonemia is complex，such as urea cycle disorders，organic acidemias，acute or chronic inflammatory liver diseases and drug-induced liver damage. Ten genetic disorders are known as the causes of primary urea cycle defect，and hyperammonemia is the main manifestation. The patients of organic acid metabolic disorders，such as methylmalonic acidemia，propionic acidemia and isovaleriuria，are often complicated with metabolic acidosis and hyperammonemia at the acute stage. The patients of mitochondrial fatty acid metabolic disorders and hyperinsulinemic hyperammonemia syndrome are usually presented with hypoglycemia and hyperammonemia in the acute phase. Early detection of hyperammonemia，etiological diag－nosis and precise treatment are keys to improve the prognosis and the quality of life of patients and their families.