Objectives Mild cognitive impairment (MCI) patients show decreased semantic memory-processing ability including that for naming objects. We examined event-related potentials (ERPs) associated with the semantic processing of MCI patients and compared their ERPs to those of cognitively normal elderly subjects. Methods A total of 25 MCI patients and 23 age-matched cognitively normal elderly controls (CN) participated in an object-word matching task that consisted of responding to whether a word stimulus matched the picture stimulus preceding it. There were three types of object-name pairs: 1) matched pair, 2) related but mismatched pair, and 3) unrelated mismatched pair. We compared the mean amplitude of ERPs to examine the mismatch effects between matched and mismatched pairs (N400M) and the category effect between inner category and outer category mismatched words (N400C) in CN and MCI patients. Results Compared to the CN subjects, the MCI patients showed a distinct N400M response to mismatch conditions, but the observed range was limited in the centroparietal area. While CN individuals showed a significantly larger N400C amplitude for unrelated mismatch pairs than those for related mismatch pairs, MCI patients exhibited no significant difference between those pair types. Conclusion The MCI showed a lack of the category effects in ERP responses, which may reflect latent behavioral impairments.