There have been no satisfactory analyses of tensification phenomena in explaining the behavior of word-initial tensification of English loanwords in Korean. The purpose of this paper is to examine tensification phenomena of English loanwords used by Korean speakers focusing on word-initial stop consonants. For this purpose, we propose the notion of strong and weak vowels, and argue that English /b, d, g/ are perceived by Korean speakers as the tense stops /p’, t’, k’/ before the strong vowels, /a, ae, ʌ, o/. Meanwhile, they become the lenis stops /p, t, k/ before the weak vowels, /i, e, ɯ, u/. This tendency can be interpreted as a hyperbolic perceptual strategy to enhance the distinctiveness of Korean stop consonants from English in the word-initial position. This matching of strong with strong and weak with weak between consonants and vowels constitutes another kind of co-articulation process. Thus we suggest a new vowel classification (i.e. strong and weak) is necessary in Korean phonology in addition to the Light and Dark vowel classification used for explaining vowel harmony and ideophones.