This study investigated the production patterns of vowel epenthesis/deletion in Korean learners’ English contrasts of /ʧ/ vs. /ʧi/ and /ʤ/ vs. /ʤi/ in word-final position, where such errors are frequently observed among the learners. Through a production experiment of 20 pairs of English words contrastive in word-final vowel, a total of 800 tokens were obtained from 20 Korean learners of English of high and low proficiency levels. The results revealed that the learners exhibited phonological processes of vowel epenthesis or deletion and that multiple factors such as voicing of a preceding consonant, frequencies of target words and learners’ proficiency in target language were involved. Voicing of a final palato-alveolar affricate played an important role in vowel epenthesis. With regard to vowel deletion, learners with low proficiency levels showed the influence of voicing of a palato-alveolar affricate by deleting the vowel /i/ more frequently after the voiceless /ʧ/ than after the voiced /ʤ/. Overall, the vowel epenthesis error seems to be motivated by L1 phonology while vowel deletion error was influenced by word frequencies. Further acoustic analysis revealed that higher-level learners’ epenthetic vowels, when compared to lexical vowels, exhibited significant differences with regard to vowel ratio and F1 while the low-level learners’ did not.