Abstract: Objective To explore the diagnosis and treatment of medium chain acyl-CoA dehydrogenase deficiency (MCADD). Methods The clinical symptoms, laboratory examination and gene detection in one case of MCADD was analyzed retrospectively. The related literatures were reviewed. Results A male child aged 3 years presented transient hypoglycemia, hyperammonemia, and liver dysfunction. Tandem mass chromatography detected acetyl carnitine and multiple acyl carnitines increase in blood. However, the gas chromatography-mass spectrometry showed normal results in urine. Gene testing found a homozygous mutation in acyl-Coenzyme A dehydrogenase (ACADM), c.572G > A p. (Trp191*). The hepatic pathology showed mild damage in hepatocyte, with degrees II inflammation and degree I fibrosis. After treatment of protecting liver and reducing enzyme, and carnitine supplementation, as well as the diet with high carbohydrate, high protein, and low fat, the liver function of the patient was back to normal. Conclusion The use of tandem mass chromatography in blood and gene detection could make a diagnosis of MCADD. Supplement of high energy nutrients and carnitine were benefit to prevent the progress of MCADD.