In order to clarify the psychological processes reflected in N400, this study explored the effects of relatedness proportion( the percentage of related prime-target pairs in a list) on the scalp distribution of N400 to auditory stimuli. Sixteen participants performed the lexical decision task with auditory words or nonwords under conditions of high and low proportion of semantically related prime-target pairs. The results showed that behavioral semantic priming effects were greater when the relatedness proportion was high compared to when it was low. Though the semantic priming effects of N400 were clear at almost all recording sites, no statistically reliable change of the amplitude or the distribution of N400 due to the relatedness proportion was confirmed. Possible differences between visual and auditory N400s and manipulations for decomposing subcomponents of N400 were discussed.