This study examines the effects of asymmetrical mappings of L2 sounds to L1 sounds on real-time processing of L2 phonology. L1-Korean participants completed a self-paced listening (SPL) task paired with a picture verification (PV) task, in which an English sentence was presented word by word along with a picture that matched or mismatched the sentence. In the critical region, an L2 vowel was deliberately replaced with the wrong vowel for two types of English vowel pairs: Type 1: English vowel pairs showing a one-to-one mapping to Korean counterparts (e.g., English: /i/ and /ae/ to Korean /i/ and /ae/, respectively); and Type 2: English vowel pairs showing a two-to-one mapping to a Korean counterpart (e.g., English /i/ and /?/ to Korean /i/). We analyzed response times (RTs) and PV accuracy. Longer RTs were observed for Type 1 errors than Type 2 errors, indicating lower sensitivity to L2 vowels with two-to-one mapping to an L1 vowel. Also, PV accuracy was lower for the sentences containing Type 2 errors. These results suggest that asymmetrical L2-L1 sound mapping can affect learners' processing of L2 phonological knowledge, which in turn can negatively affect their comprehension.