Muscle tissue, skeletal muscle as well as cardiac muscle, is commonly affected in mitochondrial disorders. One explanation for this observation is that muscle tissue has a high-energy demand and therefore is more sensitive to a deficiency of mitochondrial energy production than some other tissues. In mitochondrial disorders, skeletal muscle tissue may be affected primarily by defective respiratory chain function or secondarily to peripheral neuropathy with neurogenic muscle atrophy. The clinical manifestations of mitochondrial myopathies are variable and include muscle weakness, exercise induced cramps ad myalgia. Also, ptosis and progressive external ophtalmoplegia are typical but not obligate finding. Hereby we wanted to report a case of mitochondrial myopathy, diagnosed by histochemical and electron microscopic studies for the first time in Iran. Our case was a 12-years old girl who referred due to muscle weakness to our center which started at an age of 8 years. Later, she also developed ptosis. EMG studies were inclusive and muscle biopsy revealed typical red ragged fibers with special staining. By electron microscopy, typical mitochondrial changes were detected.