The Spirit Dance ritual is performed during the appeasement ritual to the ancestral spirits. The similarity in the ritual steps between the Mon in Thailand and in the Mon State in Myanmar was hypothesized that the Mon immigrants brought the ritual to Thailand. The ritual is conducted when someone has violated social taboos or when a promise to ancestral spirits has been fulfilled. The ritual is accompanied by a Mon ensemble. This investigation focused on the music of the Boontham Ensemble. It was found that there were 24 pieces which were related to the steps of the ritual. For the study of the music properties, five pieces were selected and it was revealed that two groups of melody were used: C D E G A and G A B D E, with the former being used most frequently. The two groups of melody were used in Abakhan, Le Anachuk and Le Khro. The latter group was used in Le Saeng and Le Krap. The ritual has been passed down through the ethnic beliefs, rules and taboos. Socio-economic changes have affected the lifestyle and livelihood of the ethnic group. To a negative extent, the riaul has rarely been performed at the community in recent times.