Abstract: Objective　Mitochondrial DNA depletion syndrome is a rare autosomal recessive disorder characterized by complex genetic and clinical manifestations. This study aimed to investigate the clinical and laboratory features of a boy with mitochondrial encephalomyopathy caused by SUCLG1 mutation. Methods　The clinical data and genetic test of a patient with mitochondrial DNA depletion syndrome were retrospectively analyzed. Result　The proband presented with limb weakness at the 4th month after birth, and presented dystrophic appearance, muscular hypotonia, psychomotor retardation, failure to thrive, hearing impairment, scoliosis, thoracocyllosis and facial features at 9 months old. Laboratory tests showed blood lactic acid and pyruvate increased, liver damage and abnormal myocardial enzymes. Plasma carnitine ester profiling showed that amino acids decreased and C4-dicarboxylic-carnitine increased. Urinary organic acid analysis showed increased concentration of methylmalonic acid and its metabolites indicating methyl malonic aciduria. MRI showed bilateral T2 hyperintensities in bilateral caudate nuelei and lenticular and brain atrophy-like changes. Brainstem auditory evoked potential showed severe hearing loss. His development quotient was 35. Genetic sequencing of MUT, MMAA, MMAB and other classic mitochondrial disease related genes of the proband revealed no mutation. Two heterozygous mutations, c.961C>G and c.713T>C, inherited from the phenotype of normal parents were detected in his SUCLG1 gene. The copy number of mitochondrial DNA was 244/cell in peripheral blood leukocytes, equivalent to 68.4% of that in normal control. Conclusion　In this study, an infant with muscular hypotonia, psychomotor retardation, deafness and slightly increased urine methyl malonic acid was diagnosed by genetic test. For patients with unexplained hypotonia, mental retardation, abnormal movements, hearing disorder together with increased blood pyruvic  acid and lactic acid , mild methylmalonic acidemia and abnormal acylcarnitine, mitochondrial DNA depletion syndrome should be considered. Gene analysis is important for diagnosis and prenatal diagnosis of the next pregnancy.