The present invention aims at providing a method of stabilizing reduced coenzyme Q 10 useful as food, food with nutrient function claims, food for specified health use, nutritional product, nutritional supplement, animal drug, drink, feed, cosmetic, pharmaceutical product, therapeutic drug, prophylactic drug and the like, as well as a composition thereof. The present invention provides a method of stabilizing reduced coenzyme Q 10 , including achieving the co-presence of reduced coenzyme Q 10 and reduced coenzyme Q9 and/or reduced coenzyme Q11, and a composition containing reduced coenzyme Q 10 and reduced coenzyme Q9 and/or reduced coenzyme Q11.