The purpose of this study is to analyse lexical and sound changes that occur in the Ubon dialect between two age-groups, the senior group over 50 years of age and the teen-age group between 11 to 19 years of age. The data, based on 23 lexical sets of 486 words, was collected by means of interviewing 60 informants from 10 amphurs in Ubon Province. In each amphur 6 male and female informants were chosen, three of whom belong to the senior group, and the rest to the teen-age group. The results are as follows :- 1. The data of 486 lexical items can be classified into 6 categories.1.1 107 words in the Ubon dialect are still in use by both groups of informants.1.2 100 words of the Ubon dialect are used by the senior group, while the teen-age group use borrowed words from the Central Thai dialict.1.3 241 words of the Ubon dialect are used by the senior group, while the teen-age group use both the words in the Ubon and the Central Thai dialects.1.4 12 words of the Ubon and the Central Thai dialects are used by the senior group, while the other group use the same lexical items borrowed from the Central Thai dialect.1.5 37 words in the ubon dialect mixed with borrowed words from the Central Thai are used by both groups.1.6 3 borrowed words from the Central Thai dialect are used by both groups. 2. Four types of lexical change are found :-2.1 new words borrowed from the Central Thai dialect.2.2 the change in the initial position of words to make them sound like the Central Thai dialect.2.3 the change in the final position of words to make them sound like the Central Thai dialect.2.4 addition of a syllable in words of the Ubon dialect to make them sound like the Central Thai dialect. 3. Two types of sound change are found :-3.1 consonant change3.2 vowel change According to this study, the Ubon dialect is apparently changing, especially in lexical words. It was found that 379 words of the Central Thai dialect were borrowed and mixed with the Ubon dialect.