Objective　To investigate the clinical and molecular genetic characteristics of hypermethioninemia caused by methionine adenosyltransferase deficiency. Methods　The clinical data and related gene analysis of hypermethioninemia caused by methionine adenosyltransferase deficiency in 3 children were retrospectively analyzed. The core pedigree analysis was carried out. Results　Three children (2 boys and 1 girl) aged from 5 months to 3 years, were from 3 unrelated families. All of them had no family history. One case was found in neonatal screening. One case was onset with pathological jaundice at 1 month old. Another case was found due to tremor and growth retardation at 2 years old. Blood amino acid ester acyl carnitine spectrum analysis showed that all of them had significantly elevated levels of methionine at 134.50-790.67 μmol/L. All children had MAT1A mutation in methionine adenosyltransferase gene. One case was heterozygous mutations with third exon c.274T>C and seventh exon c.895C>T mutation; one case had sixth exon c.757G>A homozygous mutation; and another case had seventh exon c.791G>A homozygous mutation. The core pedigree analysis showed that the mutations were from theirs parents respectively. Conclusions　For children with neurologic impairment, methionine metabolic disorders should be considered. Blood amino acids and gene analysis are important methods for confirmation of the diagnosis. Neonatal screening is an effective way to detect this disease.