Abstract： Objective To investigate the incidence and clinical characteristics of mitochondrial tRNA Leu(UUR) gene at nucleotide 3243 A to G mutation in diabetic patients in Dalian District of China. Methods According to ADA(1997)/WHO(1999) diagnostic criteria of diabetes mellitus (DM), 629 patients with DM were screened by polymerase chain reaction and restriction fragment lenghth polymorphism. Genetic and clinical analyses were performed in unrelated patients with the mutation in mtDNA and their family members. Results Six unrelated subjects (probands) with the mutation were detected. Then 6 diabetes patients and 1 IGT from the first degree relatives of these 6 probands were also identified being with the mutation, in which 11 patients are associated with sensory hearing loss and 5 patients requires insulin therapy due to secondary failure to oral hypoglycemic agents. All these patients had lower weight, were younger at diagnosis and most of them were maternally inherited. Conclusion The mutation at nucleotide 3243 in the mitochondrial tRNA Leu(UUR) gene is an important cause of diabetes in Dalian District of China. Compared with the DM patients without the mutation, the clinical characteristics associated with this mutation are young at onset, non obese, maternally inherited and deafness, et al.