The present study investigates the effect of pronunciation training on Korean adult learners’ perception and production of English vowels, /i/, /I/, /u/, and /Ω/. The study examines 1) the effect of pronunciation training on perception and production and 2) the maintenance of the effect over a one-month period. The subjects were 10 Korean graduate students who took a 4-week long English pronunciation training course. They were tested before and after pronunciation training and tested again one month after the training. In each test, both perception and production of the vowels were tested to see if the subjects were able to distinguish tense and lax vowels. In the perception part, subjects listened for words and checked the vowel (either tense or lax) sound they heard. For the production part, subjects read the selected words which contained the target vowels. The results show that pronunciation training has an effect, which supports the teachability of phonology in adulthood; however, the maintenance of the effect demonstrated a difference between perception and production, in which the effect was maintained in production but not in perception. Based on the results of the study, implications are discussed.