Rueam Ma Muad ritual is a type of alternative medicine related to the beliefs of the Thai- Khmer ethnic group. Musical performance is used as an importance instrument in ritual practice. At present, this ritual still plays an important role in the way of life of The Thai- Khmer ethnic group. This research aimed at investigating the background of Rueam Ma Muad ritual and music therapy in this ritual, current situations and difficulties concerning music therapy in Rueam Ma Muad ritual, and a pattern of perpetuation and development of music therapy in Rueam Ma Muad ritual of the Thai-Khmer ethnic group in the area of the 3 province; namely Si Sa Ket, Surin, and Buri Ram. A sample was selected by means of a purposive sampling. It divided into 3 groups: a group of 12 key informants, a group of 30 casual informants, and a group of 50 general informants. The instruments used for gathering data were a structured interview, a non-structured interview, a participative observation, a non-participative observation, and a focus group note-taking. A triangulation technique was used for analyzing data. Research results were presented by means of a descriptive analysis. The results have shown as follows: The background concerning Rueam Ma Kuad ritual and music therapy in this ritual of the three research areas is related to the belief in spirits of ancestors called "Khong Rak Sa" or "Khru Kam Nerd" (guardian spirits). The Thai-Khmer believe that each person has an own guardian spirit who can make the person ill or well according to one's good or bad behavior. The sound of music plays a role as a medium between the persons and the guardian spirit in Rueam Ma Muad ritual so musical performance is continued throughout the ritual. Music therapy in this ritual is not clear in case of when it begins but it has been practiced from the past to the present. Current situations show that Rueam Ma Muad ritual still plays an important role as medical treatment of the Thai-Khmer ethnic group and the sound of music is a main medium which links a healer, a sick people, and a Khru Kam Nerd (a guardian spirit) together. Musical instruments used for performing in this ritual divided into 2 groups: a group of musical instruments for performing melodies and a group of musical instruments for performing rhythms. Three lyrics used for performing in the ritual are a lyrics of showing respect for teachers, a lyrics of inviting a guardian spirit, and a lyric of dancing for treatment. A main difficulty of this ritual is the majority of musicians are too old and lacks descendants to carry on it with a sense of traditional appreciation. A pattern of perpetuation and development of music therapy in Rueam Ma Muad ritual of the Thai-Khmer ethnic group in the research area shows that nowadays the perpetuation is not strict with the belief in Khru Kam Nerd (guardian spirits). The majority of musicians tend to love performing music themselves, they do not only perform music therapy is developed both a pattern of musical performance and a pattern of musicians' presentation but a function of treatment is still maintained. Rueam Ma Muad ritual is a type of alternative medicine which the Thai-Khmer ethnic group conserve it as a traditional treatment in order to encourage will power of sick people by using music therapy as a catalyst. This ritual has cultural value and it is useful for establishing social solidarity. © EuroJournals Publishing, Inc. 2012.