This study investigates the correlation between the proficiency of L2 learners of English and the degree of L1 Korean intervocalic voicing rule transfer on the English intervocalic voiceless stop production by Korean English learners. Korean has a phonological rule that voiceless stops become voiced between vowels (Korean intervocalic voicing rule) but this rule does not apply in English. The aforementioned difference between the phonological rule of the two languages makes it possible to predict that Korean learners with lower L2 proficiency might produce English intervocalic voiceless stops with higher degree of voicing than the learners with higher L2 proficiency. The speech data of Korean L2 learners of English is extracted from GenieSpeeCor, rated and categorized English speech corpus of Korean L2 learners of English production, and the value of intensity drop and closure duration were used as acoustic cues for voicing difference. Alveolar stop is excluded from the data analysis because the flapped /t/ sound is difficult to distinguish from the voiced sound. As a result of the current study, when it comes to the closure duration as an acoustic cue, there was no meaningful statistical relation between learners' L2 proficiency and the degree of intervocalic stops voicing. However, in terms of the intensity drop learners with the highest L2 proficiency showed a lower degree of voicing than the learners with middle and lowest proficiency.This study investigates the correlation between the proficiency of L2 learners of English and the degree of L1 Korean intervocalic voicing rule transfer on the English intervocalic voiceless stop production by Korean English learners. Korean has a phonological rule that voiceless stops become voiced between vowels (Korean intervocalic voicing rule) but this rule does not apply in English. The aforementioned difference between the phonological rule of the two languages makes it possible to predict that Korean learners with lower L2 proficiency might produce English intervocalic voiceless stops with higher degree of voicing than the learners with higher L2 proficiency. The speech data of Korean L2 learners of English is extracted from GenieSpeeCor, rated and categorized English speech corpus of Korean L2 learners of English production, and the value of intensity drop and closure duration were used as acoustic cues for voicing difference. Alveolar stop is excluded from the data analysis because the flapped /...