Adult onset type II citrullinemia is an autosomal recessive disorder of the amino acid metabolism caused by a deficiency of liver specific argininosuccinate synthetase activity. This disease can occur at any age in life with recurrent episodes of neurological signs and symptoms such as disorientation, abnormal behaviors (aggression, irritability and hyperactivity), seizures, coma and potential death from brain edema, which are resulted from hyperammonemia. We should consider this rare metabolic disease for the adult patient who exhibits mental change and hyperammonemia without liver or brain disease. Recently. SLC25A13 gene, encoding the mitochondrial aspartate glutamate carrier protein named citrin, is demonstrated to be responsible for adult onset type II citrullinemia. We experienced a 39-year-old female who suffered from generalized weakness, dizziness and lethargy, and diagnosed as adult onset type II citrullinemia by highly elevated plasma citrulline and ammonia and the SLC25A13 gene mutation. We report here on this unusual case of adult onset type II citrullulinemia with a brief review of the related literature.