This study aims to explore the production of British English monophthongs by L2 Thai learners using an impressionistic study as well as to investigate the relationship between the target-like production of these monophthongs with vocabulary size, sex of speakers, target vowel, and L2 experience. Seventy-eight L2 Thai learners in Thailand produced 11 British English monophthongs /i(SIC), I, e, ae, (SIC), alpha(SIC), (SIC), (SIC), u(SIC), Lambda, and (SIC)/ in the /b_t/ context. They were also tested for their English vocabulary size. The impressionistic study showed that participants had no difficulty producing most target vowels /alpha(SIC), i(SIC), u(SIC), (SIC), ae, e, Iota, and Lambda/ but found the other three vowels /(SIC), (SIC), and (SIC)/ difficult. Results also showed that vocabulary size, sex of speakers, and L2 experience did not have a significant correlation with the target-like production of the monophthongs but did have an effect on target vowels. The implications of these results were discussed in the context of L1 positive transfer, the influence of spelling on the tested word, the influence of lip rounding of the vowels on the production of the monophthongs, and word frequency.