Abstract： Objective To report the clinical features of mitochondrial disease caused by mitochondrial DNA (mtDNA) A8344G point mutation.Methods We analyzed the clinical presentations and muscular pathological changes in 10 patients with genetically confirmed mtDNA A8344G point mutation.Results Among them,6 patients presented as juvenile-onset myoclonic epilepsy with ragged red fibers (MERRF) syndrome,2 suffered infant-onset Leigh syndrome and the remaining 2 were diagnosed as limbgirdle mitochondrial myopathy.The mtDNA A8344G mutation load from muscle samples showed that patients with Leigh syndrome ＞ MERRF syndrome ＞ mitochondrial myopathy (87.2％,88.4％ ＞ 69.0％-86.8％ ＞67.2％,58.4％).Conclusions Mitochondrial disease caused by A8344G point mutation shows a great heterogeneity.The mutation load of muscle mtDNA might be associated with the severity of clinical phenotype,the higher mutation load,the more severe clinical presentations.