Introduction: Recessive congenital methemoglobinemia (RCM) caused by CYB5R3 deficiency due to the mutations in the reduced nicotinamide adenine dinucleotide (NADH) cytochrome b5 reductase (CYB5R) gene is an autosomal recessive inherited disease. Clinically, it can be divided into two types, namely red blood cell affected type (RCM I) and systemically affected type (RCM II)., Case Presentation: A 5-year-old male patient was diagnosed with cyanosis for 5 years. Physical examination showed cyanosis in areas such as the lips, fingers, and toes. Laboratory examination revealed low pulse oxygen saturation (81%) and increased blood methemoglobin (23.6%). Gene testing revealed the compound heterozygous mutations in the CYB5R3 gene, c.149G>A (p.Arg50Gln) and c.331A>G (p.Lys111Glu), respectively originating from his parents. By constructing 3D models of CYB5R3 wild-type and mutant types using SWISS-MODEL software, it was found that the mutation caused significant structural abnormalities in the CYB5R protein. The relationship between CYB5R3 gene mutation sites, amino acid change, enzyme activity, and methemoglobinemia type I and II were listed and analyzed., Conclusion: A case of congenital RCM type I caused by compound heterozygous mutations in the CYB5R3 gene was reported, with c.331A>G (p.Lys111Glu) being the newly reported mutation. The homozygosity or heterozygosity of CYB5R3 gene mutations that lead to premature termination, loss of exons, and change in amino acid properties in FAD or NADH binding domains, is positively correlated with the severity (from type I to type II) of methemoglobinemia., (© 2024 S. Karger AG, Basel.)