In the present study, we examined the effects of orthographic and phonological neighborhood sizes for Japanese Katakana words using a lexical decision task. Kawakami (2002) reported an inhibitory orthographic neighborhood size effect along with a null phonological neighborhood size effect in his lexical decision tasks. In contrast, Grainger, Muneaux, Farioli, and Ziegler (2005) reported an interaction between orthographic and phonological neighborhood sizes in a lexical decision task. Therefore, we re-examined the effects of orthographic and phonological neighborhood sizes for low-frequency Katakana words in a lexical decision task. Consistent with Grainger et al., we found the interaction between orthographic and phonological neighborhood sizes, indicating that lexical decision performance for Katakana words is modulated by the nature of orthographic-phonological relationships.