Mencius’ thought, which forms a connecting link between the preceding Confucius and the subsequent Xun zi, is an important stage of development of pre-Qin Confucianism. It not only has important influence on Chinese politics, economics, culture, etc., in Chinese history, but also has far-reaching influence on Eastern and Western culture. Mencius’ thought is distilled in his book Mencius. Nihonkoku Genzaisho Mokuroku (The Catalogue of Chinese Texts Existent in Japan) comprises 17,209 volumes written in middle of the Japanese Heian period, including 1568 ancient books from the Tang Dynasty. The Eastern Han Dynasty scholar, Zhao Qi’s Commentaries on Mencius Vol.14 can be found in this catalogue, confirming that Mencius had been introduced to Japan before the Middle of the Heian period. It is generally accepted in present Sino-Japanese academic circles that Mencius as part of the classics of Confucianism was spread widely after the inception of the Edo period. But, from reading Nihon Shoki, the oldest officially authorized history of Japan extant in the present day, the author finds that there are many elements of Mencius’ thought contained in it. Nihon Shoki was finished in 720 C. E., yet its compilation can be traced back to the 10th year of the rule of the Emperor Tenmu (681 C. E.). It has a strong political color and obvious external awareness. After the victory in the Jinshin War, he Emperor Tenmu set up the régime, which attempted to user the book to defend the legitimacy of the Imperial family, and declared Japan’s existence as a nation that was as developed as the neighboring countries of China and Korea. Against this background, the descriptions of each Japanese Emperor and their related political activities in Nihon Shoki are much influenced by Chinese history. Traces of Mencius’s thought such as “benevolent governance”, “people-oriented”, “the Tang-Wu revolution (the expulsion of disqualified monarchs by King Tang and King Wu)”, etc., can also be discerned in Nihon Shoki.