Objective To investigate the clinical, electrophysiological, and genetic features of one family and one sporadic patient with congenital myotonia (MC).Methods Detailed clinical data were collected from pedigree and the sporadic patient with congenital myotonia, and then chloride channel protein 1 (CLCN1) and SCN4A genes were sequenced for the proband and related relatives in pedigree.Results There were 7 patients of 3 generations in pedigree, among whom 5 patients and 1 asymptomatic family member received genetic test, and the results showed that the 5 patients carried CLCN1 A298T mutation. The S723R missense heterozygous mutation in CLCN1 was found in the sporadic patients.Conclusions A298T mutation in the CLCN1 gene is the pathogenic mutation for pedigree, and further studies are needed to clarify whether S723R is a pathogenic mutation for the sporadic patient.