The present invention relates to a method of efficiently producing reduced coenzyme Q.sub.10 having excellent qualities which is useful as an ingredient in foods, functional nutritive foods, specific health foods, nutritional supplements, nutrients, animal drugs, drinks, feeds, cosmetics, medicines, remedies, preventive drugs, etc. This method is suitable for industrial production thereof.It is possible to handle reduced coenzyme Q.sub.10 in state of being protected from oxidation by molecular oxygen by bringing the reduced coenzyme Q.sub.10 in contact with a solvent containing a strong acid. Furthermore, when reduced coenzyme Q.sub.10 is crystallized in the presence of a strong acid, crystallization can be carried out while the formation of oxidized coenzyme Q.sub.10 as a by product is minimized, and, then high-quality crystals thereof can be produced.