The neural correlates of "feeling-of-not- knowing" ("FOnK", i.e. the feeling-of-knowing judgments that accurately predicted "not knowing" or "misses" in the criterion test) were investigated by the event-related fMRI method through an RJR (recall-judgment-recognition) procedure that adopted unrelated word pairs as materials. Results revealed that, relative to the inaccurate "FOnK" predictions, the accurate ones were associated with activities in right ventral prefrontal cortex (PFC) and insula, the areas that were known to subserve "cue specification" in which the retrieval cues were converted into "descriptors" that could be used for direct memory search. This result implied that the accurate "FOnK" predictions relayed more on "cue specification" process than the inaccurate ones and was in consistent with the cue familiarity heuristic hypothesis of feeling-of-knowing.