Medium-chain acyl-coenzyme A dehydrogenase deficiency (MCADD) is an autosomal recessive inborn error of mitochondrial fatty acid β-oxidation, caused by mutations in the ACADM gene. As it is the most commonly inherited disorder of the mitochondrial fatty acid oxidation in Caucasians, there are no related reports in China diagnosed by molecular genetic testing. We report here the case of a 2-year-old female patient who had hepatomegaly and abnormal liver function with a common illness, and who had been healthy before. A marked increase found in the concentration of C8-carnitine with the help of tandem mass spectrometry (MS/MS) profile, as well as the presence of hexanoylglycine and cyclohepta acyl glycinate as shown in the urinary gas chromatography/mass spectrometry (GC/MS) were suggestive of MCADD, a diagnosis that was confirmed by genetic analysis that showed compound heterozygosity for a missense mutation, c.362C>T(p.Thr121Ile), and a 4-bp deletion, c.448-453delCTGA, in the medium-chain acyl-coenzyme A dehydrogenase (MCAD) gene, also named ACADM gene. There are no related reports in China. This report broadens the phenotype and genotype of MCADD in China and underlines the difficulty of diagnosis.