The diet and feeding practice of ancient Japanese horses revealed by stable carbon isotope analysis will be discussed along with a brief comparison with the preliminary results from the Three Kingdoms period horses in Korea. The carbon isotope ratios of horses from western to the northeastern Japan exhibits a significant regional variation and the proportion of C4 plant diet was significantly high in the inland area of eastern Japan. Regarding the change according to age in individuals, different patterns were detected such as consistent C3 diet, consistent C3/C4 mixed diet, and a shift from C3 diet to mixed diet at around 3 years old. In mainland Japan, it has been confirmed in modern specimen that horse diet in free grazing state is composed mainly of C3 plants. The high proportion of C4 plants would therefore indicate an artificial feeding of millets. This is consistent with ancient documents stipulating the feeding of millet to fine horses. The existence of diet shift around 3 years old is also in congruent with the provisions regarding the age of selection in horses. Changes in diet could also be a clue to studying the mobility of horses, as the regional variation in the intake of millets was also confirmed. Study of horse diet and feeding practices by carbon isotope analysis can be an effective tool in reconstructing the ancient relationships between Japanese archipelago and the Korean peninsula through horse breeding system.: Horse feeding strategy in ancient Japan and Korea: Stable carbon isotope analysis of the tooth enamel