This paper first reviews the recent condition of historical source materials regarding the Malacca straits during the Yuan period (1276-1368), materials such as 1) the Tamil inscription of Quanzhou, 1281, which was discussed by Karashima Noboru (1988) as evidence of revitalization of the Tamil diaspora community, 2) Da-de-nan-hai-zhi (1304), a local gazetteer of Guangzhou which began to be used by historians in 1986, 3) the Yajima Hikoichi's Japanese translation of the Travels of Ibn Battuta with rich annotations (8 volumes, 1996-2002), and 4) Nomura Toru's Japanese translation of Hikayat Raja Pasai (2001). Concerning the authenticity for Jacob d'Ancona's the City of Light as a historical source, the author ends up supporting the group that assumes the book to be a forgery, mainly because Jacob claims that he crossed the Bay of Bengal from East to West during May and June, which would have been during high tide of the Southwest monsoon.In striking contrast to the Song period (960-1276), when San-fo-qi was frequently referred to as one of China's most important tributary countries and a country that occupied the crossroads of the South Seas, San-fo-qi is mentioned only three times during the Yuan period, in Da-de-nan-hai-zhi, Dao-yi-zhi-lue and Yuan-shi. Yuan-shi mentions it only once (volume 11), while its “Accounts of Foreign Countries” (volumes 208-210) makes no mention of any country in the Malacca straits area. It is as if Yuan-shi does not recognize any country in the South Seas between Campa in Southeast Asia and Ma' bar in South India. The Yuan central government did not recognize San-fo-qi, which, nevertheless, was recognized or at least remembered by local societies with close relations to the South Sea countries.Soon after the fall of the Song dynasty, followed by the absorption of maritime power in South China by the Yuan dynasty in 1276, friction began to occur between the central government and local sea merchants in Fujian, among whom the descendants of Da-shi (Arab or Persian), such as Pu-shou-geng, were influential. After a compromise was reached, San-fo-qi was mentioned by the latter in 1280, who wanted to manipulate the tributary mission from San-fo-qi, like they frequently did under the Song Dynasty. However, their proposal was rejected by the central government, which now wanted to participate directly in the South Sea trade to capture the lion's share of the profits.The Yuan central government had no reason to recognize San-fo-qi as a tributary country, since it was not a single polity, but rather a general name for the tributary countries of the Malacca straits area (Fukami 1987 and 2001). This is why many “San-fo-qi countries, ” such as Samudera and Malayu were recognized and recorded by the Yuan dynasty.It is well-known South India was the most important emporium in the South Seas (South China Sea-Indian Ocean region) during the Yuan period. We have enough sources to show conclusively that more people and ships from China and Southeast Asia frequented South Indian ports at that time than during the Song period. This development seems to be the result of (or the reason for) the technical progress that was achieved in the art of navigation. At the time of Ling-wai-dai-da (1178) and Zhu fan-zhi (1225), two winter monsoons were required to sail from South China to South India, because ships that departed China in the winter monsoon had to wait for the next winter monsoon at North Sumatra.