This case study investigated whether an experiment group of thirty-two Korean college students well corresponded to the height of back vowel /ɔ/, the lip-rounding of /ɒ/ and /ɑ/, and the vowel merging of cot - caught as the criteria for telling British from American English. This research finds its meaning in elucidating Korean college students’ perceptual gap or misconception in discriminating British English from American English within the boundary of English vowel height and roundness, and bring to light the possible background reasons. About the vowel /ɔ/, their judgement was seriously hurt because their perception of the vowel height was negatively influenced by non-linguistic factors like the preoccupation about a particular English. On the other hand, they well adopted and used the lip-rounding difference between /ɒ/ and /ɑ/ as a criterion. However, they failed to use the vowel merging of cot and caught as another criterion because most of the Korean students had not fully familiarized themselves with the vowel merging.