The aim of this paper is to analyze vowels in voice imitation and disguised voice, and to find the invariable phonetic features of the speaker. In this paper we examined the formants of monophthongs /a, u, i, o, {$omega},{;}{varepsilon},{;}{Lambda}$/. The results of the present are as follows : $circled1$ Speakers change their vocal tract features. $circled2$ Vowels /a, ${varepsilon}$, i/ appear to be proper for speaker recognition since they show invariable acoustic feature during voice modulation. $circled3$ F1 does not change easily compared to higher formants. $circled4$ F3-F2 appears to be constituent for a speaker identification in vowel /a/ and /$varepsilon$/, and F4-F2 in vowel /i/. $circled5$ Resulting of F-ratio, differences of each formants were more useful than individual formant of a vowel to speaker recognition.