In experimental paradigms, voluntary orienting of visual-spatial attention is conventionally achieved through the Posner task in which predictive central cues are presented to indicate the location of an upcoming peripheral target, followed by varying inter-stimulus interval (ISI). Previous studies have indicated that the effects of ISI on spatial attention can occur. However, to date, brain mechanisms associated with ISI effects remain unclear. We investigated the brain activity changes along with increased ISI. Behaviorally, subjects exhibited no difference in the different ISI conditions. However, neuroimaging data revealed reduced activity in the posterior cingulate cortex (PCC) as ISI becomes longer. We propose that, as time went on, the strength of visual-spatial bias reduced, resulting in decreased PCC activation.