Towards the close of the nineteenth Buddhist century which witnessed the decline of Sukhothai kingdom, King U-thong of Suphunaphum, once under Sukhothai domination, proclaimed his state as independent of Sukhothai power and built up his capital at a town called Sri Ayutthaya, south of Sukhothai. This kingdom, which lasted 417 years, are ruled over by 33 kings.

Through more than four centuries which marked the age of Ayutthaya kingdom, Theravada Buddhism in Thailand seemed to reach its zenith of popularity. Within and without the city of Ayutthaya there scattered innumerable temples and pagodas which served as places, thereby exerting a great influence on the spiritual life of the people. Buddhist art, both in the field of architecture and Buddha-image construction, were on the same line of flourishing. An illustrative example of this fact may be seen today in the temple of the Foot-Prints at Saraburi. There was also a tradition which is still in practice today for every Thai young man to be ordained at least once as a bhikkhu. Several kings such as Pra Borom Trai Lokanatha, the 18th king, in following the example set by King Li-Thai of Sukhothai period, had temporarily renounced his throne to be ordained as a bhikkhu.

During the reign of Phra Borom-Kote, the thirty-first of Ayutthaya kingdom, there reigned in Ceylon a king named Kitti-Siri-Raj-Singha, who being discouraged by the decline of Buddhism in his island country and learning that Buddhism was purer in Thailand than any other country, sent forth his religious mission to the Thai King, asking a favour of some Thai bhikkhus to revive the spirit of Theravada Buddhism which had almost died out in his land. This was a good occasion when Thailand was able to repay her debt to Ceylon and the Venerable Upali, together with his followers, were sent to Ceylon. Thus the community of Ceyl;onese bhikkhus ordained by the Thai bhikkhus at that time has ever since been called Upali-Vangsa or Siam-Vangsa. It is the well known and most revered sect in Ceylon.

Religious literature of Ayutthaya, however, abounded both in Pali and Thai language, but most of them were most regretfully destroyed when the kingdom was ruthlessly overrun by the enemy in 2310 BE.