Sukhothai is considered to be the first true Thai kingdom. (1238 till 1438)
Today many Thais view the Sukhothai period as the golden era of Thai history, an ideal state, a land of plenty, governed by just and paternal kings who ruled over peaceful, contented citizens.
Wat Mahathat completed in the first half of the 13th century lies in its present day natural setting, the greatest of the Sukhothai wats, and Wat Saphan Hin, to the west of the city, in the forest, containing a 12.5 meter high Buddha image with its hand raised in the attitide of 'giving protection', both clearly illustrate the 'Sukhothai Style'.

Example:

Wat Mahathat

Sri Sanpet

Thai Sukhothai Style Art

Thai Sukhothai art refers to the art and style of the Sukhothai [ translated as the dawn of happiness ] Empire Period. Sculpture [ bronze, stucco and stone ] was inspired by Theravada Buddhism which created a new style in which spiritual serenity is merged with human form and reflected in the numerous images of the Buddha.
Accordingly the artists created images that were intended to reflect the compassionate and superhuman nature of the Buddha.
During this period bronze images of Hindu gods were also caste. These Hindu gods are crowned and wear royal attire and were cult objects in royal court rituals performed by Brahman priests.
In addition, Sukhothai was famous for ceramics. There were two forms, the monochromes in brown and white and the celadon and painted wares. The later have dark brown or black designs and have a clear glaze.
Sculpture [ bronze, stucco and stone ] was inspired by Theravada Buddhism which created a new style in which spiritual serenity is merged with human form and reflected in the numerous images of the Buddha.