Purpose To investigate mitochondrial DNA (mt-DNA) mutations in optic neuritis of unknow reason (ONUR) and to assess the pathogenic and differential diagnostic values of screening for mt-DNA mutations in ONUR. Method Thirty patients with ONUR were screened for mt-DNA mutations by using SSCP,mutation-specific primer PCR and sequencing. Results mt-DNA mutations were found in 12 out of the thirty patients.All of the mutations were at 11778 position,but no one at 3460 and 15257. Conclusions Quite a number of patients (12/30,40%) with ONUR were caused actually by mt-DNA mutation．Screening for mt-DNA mutation in these patients has a pathogenic and differential diagnostic significance. (Chin J Ocul Fundus Dis,2000,16:78-79)