Kearns-Sayre Syndrome (KSS) is rare mitochondrial disorder characterized by chronic progressive external ophthalmoplegia, atypical retinal pigmentation and complete heart block. It is occasionally combined endocrinologic symptoms such as hypoparathyroidism, short stature, diabetes mellitus and hypothyroidism. We reported the effect of Coenzyme Q10 on total serum calcium concentration in 17 years old girl with KSS and hypoparathyroidism. The patients was treated with alfacalcidol (1alpha-OHD3), Coenzyme Q10 and oral calcium agent. Total serum calcium concentration had even remained within normal range and hypercalcemia was developed suddenly after treatment of combination of Coenzyme Q10 and alfacalcidol (1alpha-OHD3). After stop of all medication, her total calcium concentration was decreased to 7.6 mg/dL and remained in normal range with oral calcium (2 g/day) and Coenzyme Q10 (150 mcg/day) daily. The action of Coenzyme Q10 is not clearly defined but, we could explain Coenzyme Q10 activates the capacity of the patient to produce the active form of Vitamin D, 1alpha-OHD3.