The purpose of this thesis was to study the phonology of Khmu dialect and to compare lexical usage in Khmu according to different age groups spoken at Hintum village Banrai District, Uthaithani Province, including the tendencies of lexical usage among three different age groups. A tagmemic framework was used for analyzing the phonological system. This was a field qualitative research. The researcher used 803 basic words that are used in daily life divided into 12 groups for interviewing the Hintum Khmu speakers. Data were collected from 18 informants; 9 females and 9 males. They were divided into three different age groups: the first age group is 55 years old or more, the second age group is between 35-45 years old and the third age group is between 15-25 years old. The result of the phonological study shows that there are 31 consonant phonemes /p, pH, b, t, tH, d, c, cH, �, k, kH, g, /, s, x, �, h, m, Hm, n, Hn, �, H�, N, HN, l, Hl, w, Hw, j, Hj/, 19 single vowel phonemes /i, i�, e, e�, E, E�, �, ��, �, ��, ��, a, a�, u, u�, o, o�, �, ��/, and 3 diphthongs /i�, ��, u�/. There are three types of syllables: major syllable, minor syllable and presyllable. In addition, three types of phonological words were found, monosyllabic words, disyllabic words and trisyllabic words and two types of intonation groups were found: falling and rising-falling contours. This dialect has no tone and no register. It has the voicing contrast at the initial consonant. As for the study of Khmu used by the speakers of three different age groups the results show that there are 5 categories of lexical usage. 1) The usage of the same lexical items as the original word (222 words or 27.6%) 2) The usage of the same lexical items but with phonological variation when spoken by people of different age groups (268 words or 33.4 %) 3) The usage of other lexical items whose meaning are in the same semantic fields as the original word (29 words or 3.6 %) 4) The usage of both Khmu and Thai as compound words (35 words or 4.4 %) 5) The usage of loan words from Thai (249 words or 31 %) Besides the lexical variation, there are also phonological variations. There are variations in consonants, vowels and syllable structure. The variation of consonants and vowels are found mostly in the third and the second age groups respectively. The reduction and the loss of syllable are also found, especially in the third age group. The study also indicates that, the speakers of the third age group demonstrate the most lexical change and phonological variation and show a tendency of using a narrow domain of usage for the Khmu vocabularies and more Thai loan words.