<p>The current study examines Japanese loanwords from English in the framework of optimality theory (OT). The goal of this study is to investigate which vowels native Japanese-speaking borrowers epenthesize and when they delete consonants in modifying English source words. We aim to offer (i) a unified explanation with a single ranking of constraints for vowel epenthesis and (ii) a perceptual explanation for consonant deletion. Two experiments with native Japanese speakers were conducted to support the proposed analyses. The first experiment demonstrated that, when modifying illicit phonotactics in foreign words, native Japanese speakers select epenthetic vowels based on the constraint ranking of: Complex, CodaCond >> Max-IO >> Palatal-Front, SyllableInventoryStructure (SIS), Ident-IO >> *Round, *Low >> *Front >> High >> Back >> Dep-IO. The second experiment revealed that Japanese loanwords can be subject to deletion of consonants rather than vowel epenthesis, due to the lack of perceptual salience; Japanese speakers delete consonants when they fail to perceive consonants in coda and clusters in source words, especially when the source words are aurally given.</p>