This study examined the production of English monophthong vowels of native speakers of Thai. The results of the acoustic analysis of Thai English (ThaiE) monophthongs suggest the influence of Thai on ThaiE in the maintenance of length contrast between vowel pairs. The results also indicate transference of Thai vowel quality to comparable English vowels /iː/, /e/, /uː/, / ɔː/ and /æ/. One of the effects of this influence is the maintenance of the vowel contrast between /e/ and /æ/ in ThaiE. These findings lend some support to Flege's (1995) Speech Learning Model which posits that second language sounds which have phonetic similarity to those in the first language will tend to be merged. The findings contribute empirical evidence to and complement existing research on Thai English pronunciation.