Between about 1000 and 1250 the whole of central Siam was under the rule of the Khmers. It was the period of the greatest glory of the Cambodian Kingdom, the age of Angkor. Khmer art had a different basis from that of the Mons in Dvaravati. In Angkor Hinduism prevailed at first, and later Mahayana Buddhism, but never Theravada. Vast temple structures were erected in honour of the King, who was venerated as the incarnation of Shiva, Vishnu or a Bodhisattva. The cult of the God-King was the focal point of life in Angkor, the bond that held the state together. Sculpture and architecture served for the deification of the King. The world of the Hindu gods was transposed through the great temples to the human world, thereby guaranteeing the prosperity and long life of the Kingdom. Of this time in the history of Thaland little is known. A Khmer viceroy resided in the ancient Mon city of Lopburi. This city gave its name to the Lopburi style, which outlasted the period of Cambodian rule (11th – 15th century).
Pure Khmer architecture is found in a few temple compounds on the soil of modern Thailand, such as Si Satchanalai, Phetchaburi and Phitsanulok. In most cases the tower like sanctuaries (prasad) are preserved, from the form of which the Thai prang later developed. The most important Khmer monument in Thailand is the temple at Phimai, which dates from the early 12th century. In various other places, particularly Lopburi, the Khmer style had already begun to change under Thai influence.
Khmer sculpture is mainly in stone. In bronze works the clay or wax is not modelled but carved, so that clear cut details are a feature of this style. The Lopburi style differs in various points from the official Cambodian style, and the Mon inheritance from the Dvaravati period is apparent. Clearly the bulk of the Mon population remained faithful to Theravada even under the Khmers. Large numbers of Buddha images are found in the Lophuri style.