The cultural belief are a social phenomenon, it has inheritance, it always exists in the flow of the movement and development of history -society. Each ethnic group, with its historical conditions, has a traditional culture with its own unique, long-standing, and stable features, which is the national cultural identity. The cultural beliefs are a field with many special features and important values in the spiritual life of the Khmer people in the Mekong Delta. It is a combination of indigenous culture and religious culture. The article aims to highlight the cultural beliefs expressions of the Khmer Mekong Delta people through traditional festivals including Chol Chnam Thmay festival, Ok Om Bok festival, Pithi Sene Dolta festival. Traditional festivals of the Khmer in the Mekong Delta are influenced by Theravada Buddhism, so traditional festivals usually take place at the place associated with the temple, the form of the festival is also associated with the ritual of Theravada Buddhism. Their traditional festivals are deeply humane, expressed in such contents as filial piety, respect for grandparents, parents, gratitude to ancestors, living morally, living in harmony with nature. The article has pointed out the values of cultural beliefs through traditional festivals of the Mekong Delta people, it is expressed in their social life today. Therefore, research on cultural beliefs, traditional culture studies of the Khmer Mekong Delta is to study the social background, their creations, and their culture in social history. Thereby, we find out the characteristics and quintessence in the traditional value system, the nation's culture, in order to honor and promote these cultural values, and bring it to better service for the younger generation's life.