Combining clinical diagnosis with genetic analysis is considered necessary to reach accurate conclusions and select appropriate treatments for developmental and epileptic encephalopathy. Whole exome sequencing is a method of next-generation sequencing that provides information about protein-coding regions to identify disease-causing mutations. This study utilized whole exome sequencing for a Vietnamese female pediatric patient suspected of having developmental and epileptic encephalopathy at Children's Hospital 2. Through data screening, we identified a missense mutation in the conserved position of the KCNT1 gene, p.Ala934Thr. This mutation is classified as pathogenic and causes structural changes in the protein, affecting potassium channel activity and causing developmental and epileptic encephalopathy. This is a new insight into the disease-causing mechanism of the KCNT1 gene mutation, providing a basis for accurate diagnosis and appropriate treatment.