This research was designed to examine how the knowledge of kanji in their first language influences the processing of Japanese kanji-words for Korean learners. The experiment adopted a lexical decision task in which the learners’ reaction time was measured. Two experiments were conducted with learners of two different proficiency levels. The first involved kanji-words whose written forms are similar in the two languages. The second involved kanjiwords which are similar phonologically. The following results were determined: (1) Highly proficient learners showed a tendency towards shorter reaction times than the less proficient learners, but the reaction times were not dependent on whether the written forms of stimulus kanji-words were similar to those in L1; (2) Highly proficient learners showed shorter reaction times than the less proficient learners, and the reaction times were not dependent on whether the sounds of stimulus kanji-words were similar to those in L1 for learners of both levels. The results of this research differed slightly from an earlier experiment involving a naming task in which reading phonologically similar kanji-words required less time for learners of both levels. In the current experiment, no difference in reaction time was observed between the two levels in this case. This suggests that phonological facilitation in the earlier experiment occurred when the learners had access to voice output in Japanese.