Generally, there were two major factors that led to its decline: one was the class conflict caused by the heavy taxation, the other one was the ethnic contradiction resulting from the 'Four Class System'.
The Yuan court actually first began to decline during the reign of Emperor Renzong, when peasant uprisings emerged in southern China. However, despite the warning of the uprisings, corruption of the Yuan court officials continued. Also, power struggles within the ruling class became more and more serious. For instance in the short period from the beginning of Emperor Wuzong's reign in 1308 to the start of Emperor Huizong's reign in 1333, there were eight emperors. During this period, the corruption became severe as subordinate officials were commonly appointed on the basis of bribery rather than merit; the land was gradually concentrated in the hands of Mongolian aristocrats and a select group of powerful Han landlords; a fiscal crisis in the Yuan court also broke out due to the luxurious lifestyles of the ruling class. Even worse, the Mongolian army became corrupt and gradually disintegrated.
During the reign of the last emperor, the real power of the Yuan regime fell into the hands of Cheng Xiang (prime minister) named Bo Yan, who was born of a Mongolian noble family. He was rather hostile to the Han people and introduced a series of policies unfavorable to the Han. This magnified the seriousness of the ethic contradiction. Misfortunes never come singly. The Yellow River burst its banks three times in the late Yuan Dynasty. As a result, serious natural disasters broke out and the masses were forced to live in dire poverty. Under such circumstances, groups of farmers left the land and successively launched armed uprisings. Although many peasants' uprisings were successfully suppressed by the Yuan army, the corrupt regime of the Yuan Dynasty was constantly impacted by this surging wave and teetered on the verge of collapse.
The Yuan Dynasty was eventually destroyed by the peasants' uprising. In 1351, the 'Hongjinjun (army with red head-scarves) Uprising', led by a man named Liu Futong, broke out in Yingzhou (in current Anhui Province). Fierce battles took place progressively between the Hongjinjun military force and the Yuan army.