After Ramkhamhaeng's death, he was succeeded by his son Loethai. The vassal kingdoms, first Uttaradit in the north, then soon after the Laotian kingdoms of Luang Prabang and Vientiane (Wiangchan), liberated themselves from their overlord. In 1319 the Mon state to the west broke away, and in 1321 Lanna placed Tak, one of the oldest towns under the control of Sukhothai, under its control. To the south the powerful city of Suphanburi also broke free early in the reign of Loethai. Thus the kingdom was quickly reduced to its former local importance only. Meanwhile, Ayutthaya rose in strength, and finally in 1378 King Thammaracha II had to submit to this new power.
Replica of Silajaruek Pokhun Ramkamhaeng Silajaruek Sukhothai (Stone Inscriptions form Sukhothai kingdom) are hundreds of stone inscriptions that form a historical record of the period. Among the most important inscriptions are Silajaruek Pokhun Ramkamhaeng (Stone Inscription of King Ramkamhaeng), Silajaruek Wat Srichum (an account on history of the region itself and of Srilanka), and Silajaruek Wat Pamamuang (a Politico-Religious record of King Loethai).
Sukhothai became a tributary state of Ayutthaya between 1365 and 1378. In 1412 Ayutthaya installed a chief resident, and King Thammaracha IV was installed on the throne by Ayutthaya. Around 1430 Thammaracha moved his capital to Phitsanulok, and after his death in 1438 the kingdom was reduced in status to a mere province of Ayutthaya.