It is intended to provide a method of stabilizing reduced coenzyme Q<sb>10</sb> which is useful in foods with nutrient function claims, foods for specific health uses and the like; a method of preserving the same; and so on. Moreover, a process whereby reduced coenzyme Q<sb>10</sb> with good qualities can be efficiently obtained by an industrially adequate method is provided. Reduced coenzyme Q<sb>10</sb> is brought into contact with ascorbic acid or its derivative or citric acid or its derivative to thereby give a stabilized composition in which the reduced coenzyme Q<sb>10</sb> can be handled as being protected from oxidation by molecular oxygen and which can be stored stably. Further, reduced coenzyme Q<sb>10</sb> is crystallized in the presence of ascorbic acid or its derivative, etc. Thus, it can be converted into crystals while minimizing the formation of oxidized coenzyme Q<sb>10</sb> as a by−product, thereby giving reduced coenzyme Q<sb>10</sb> with good qualities. Moreover, oxidized coenzyme Q<sb>10</sb> is converted into reduced coenzyme Q<sb>10</sb> by reducing with the use of ascorbic acid or its derivative and the thus formed reduced coenzyme Q<sb>10</sb> is subsequently crystallized in the presence of the ascorbic acid or its derivative so that reduced coenzyme Q<sb>10</sb> with good qualities can be produced by a simplified procedure within a shorter period.