The invention provides a composition containing reduced coenzyme Q.sub.10, which is useful as a pharmaceutical product, food, cosmetic, animal drug, feed, and the like, as well as a stabilization method thereof. The oxidation of reduced coenzyme Q.sub.10 into oxidized coenzyme Q.sub.10 by molecular oxygen can be suppressed by setting, in a composition containing reduced coenzyme Q.sub.10 and at least one kind of oil component selected from the group consisting of fat, oil, a mixture thereof, fatty acid, wax and a surfactant, the weight ratio of the reduced coenzyme Q.sub.10 to the oil component to not less than 1/4 when the composition is a liquid, and not less than 1/9 when the composition is a solid.