Introduction: A number of maternally inherited mitochondrial diseases with distinct clinical phenotypes have been associated with point mutations in mtDNA, all of which result in neurologic or neuromuscular disorders. Several studies showed that mutations in the tRNA genes of mtDNA could cause mitochondrial disease due to the decreased synthesis of mitochondrial DNA coded proteins. Materials and methods: A patient having clinical, biochemical, and histochemical abnormalities compatible with mitochondrial disease but without rearrangements in her mtDNA, was subjected to study for point mutations in the mitochondrial tRNA genes. Using molecular biology techniques such as PCR, RFLP and sequencing analysis. Results: A new mutation in mtDNA involving tRNASer(UCN) was identified in this patinet with muscle weekness and cardiomyopathy. This heteroplasmic T to C transition at position 7480 affects the anticodon loop at a highly conserved site and was not detected in control individuals. Analysis of mtDNA from blood cells and muscle showed that the mutation was present in both tissues. Levels of the mutant mtDNA in muscle and white blood cells were quantified and showed 73% and 30% heteroplasmy in the investigated tissues, respectively. The mutation was not present in the blood cells taken from the patient's relatives. Study of cross sectioned of the muscle fibres revealed the difference in the mutant content of the COX deficient and normal fibres in this patient. COX deficient fibres harboured more than 90% mutant mtDNA while the level of mutation in the COX normal fibres ranged from 9% to 68% . Conclusion: These data suggest that this mutation could well be the cause of clinical presentation of mitochondrial disease in this patient.