This study examined the foreign accent of Korean speakers of English concerning vowel length and utterance position. It then attempts to explain the foreign accent of Koreans when they speak English. The method was to measure the sentence-initial and sentence-final vowels as spoken by Koreans. I chose these two positions, sentence-initial and sentence-final, in order to know if Korean speakers of English, compared with native English speakers, show a difference in sentence stress. I chose English diphthongs, because most Koreans have difficulty pronouncing these sounds. I found that Korean speakers of English as a second language do not know English sentence stress patterns and show a foreign accent, especially when using diphthongs.