The current study explores the effects of processing demands and proficiency on second language (L2) learners' acceptability judgment of wh-island sentences. A total of 65 adult Korean learners of English and ten native speakers (NSs) of English participated in an experiment that combined self-paced reading and acceptability judgment. They were presented with wh-island and non-island sentences that varied by whether the intervening nominal constituents inside the dependency were lexical noun phrases (NPs) or pronouns. The results showed that high-proficiency learners gave a higher rating to non-island sentences than to wh-island sentences, which is comparable to the NSs' rating pattern, whereas low proficiency learners showed a chance-level performance. The learners preferred the lexical condition to the pronoun condition, whereas the NSs exhibited the opposite preference. In the reading time (RT) analyses, both NSs and L2 learners slowed down in the lexical condition at regions where intervening NPs occurred, indicating that lexical nouns incur higher processing costs compared with pronouns. The NSs slowed down at the embedded verb region in the pronoun condition, whereas L2 learners showed no RT difference between conditions. The findings suggest that L2 learners' acceptability judgment is affected by their proficiency, but not as much by processing costs in terms of RT.