Objective To explore the clinical phenotype and genotype of infantile cardioencephalomyopathy due to cytochrome c oxidase deficiency-4 (CEMCOX4). Methods The clinical data of an infantile CEMCOX 4 was analyzed retrospectively and the related literature was reviewed. Results The 5 -day-old female infant presented with dyspnea immediately after birth. Repeated blood gas analysis showed an increase in lactic acid. Cardiac color Doppler ultrasonography showed hypertrophic cardiomyopathy with bilateral ventricular outflow tract obstruction. A homozygous mutation of c. 411 _ 412 insAAAG in COA 6 gene was detected by whole exon sequencing and the mutation resulted in the change of amino acid synthesis starting from the 140 th lysine and ending at subsequent 4 th amino acid after the change (p.Lys140 ArgfsTer 4 ). It may seriously affect the protein function. Family pedigree verification showed that both parents carried the c. 411 _ 412 insAAAG heterozygous variation. This mutation has not been reported. Literature review showed that infantile CEMCOX 4 usually presented with lactic acidosis, hypotonia and hypertrophic cardiomyopathy, and the prognosis of homozygous variation is worse than that of compound heterozygous variation. Conclusions One case of infant CEMCOX 4 was reported. A new mutation in COA 6 gene was discovered, which expanded the COA 6 gene mutation spectrum.