In order to evaluate the effect of riboflavin on thyroid hormone levels in the blood of rats during cold exposure, male adult rats were divided randomly into six or eight groups. The group 1,2,5 and 6 were supplemented with 1.5mg riboflavin per kg basal diet, but the group 3, 4, 7 and 8 with 6.0mg riboflavin per kg basal diet. The group 1, 3, 5 and 7 were kept at the cold room (4?0.5) and the group 2,4,6 and 8 at room temperature. The results revealed that supplementation of riboflavin to cold exposed rats could raise the lowered T4 concentration of serum and shorten the time of T4 returned to its original levels. Therefore, riboflayin could hasten cold acclimatization of organism.