A growing body of literature describes older adults showing decline in cognitive control (top-down executive control) that is associated with frontal lobe change. Traditional neuropsychological tests examining frontal dysfunction, such as the Stroop and the Trail Making tests, measure individual's top-down control ability by comparing the time taken to complete the task in control condition and the interference condition. Trial-by-trial intra-individual reaction time (RT) variability in cognitive control tasks, however, could also be a sensitive measure of frontal integrity, since it has been shown that frontal involvement is critical in maintaining cognitive stability by minimizing fluctuation of cognitive performance. In this study, we purported to investigate the neural correlates of intra-individual RT variability in an experimental cognitive control task, namely the Multi-Source Interference Task (MSIT). Utilizing functional magnetic resonance imaging (fMRI), we investigated the brain regions activated when healthy older adults (n=52) performed the MSIT, which consists of interference and control conditions. Regional activation that was significantly different in the interference condition compared to the control condition was examined to find brain regions involved in inhibitory control. Within the regions found to be involved in inhibitory control, we identified regions that were specifically associated with the intra-individual RT variability. Inhibitory success was correlated with smaller intra-individual variability of RT, while the magnitude of the variability was reflected in the activation of the frontal regions, such as the left middle and left inferior frontal gyri, as well as the left supramarginal gyrus. Individuals with greater intra-individual variability during the MSIT activated the inhibitory control network to a greater magnitude, possibly reflecting the increased demand for cognitive control network due to their less efficient inhibitory control system. This is consistent with previous findings that showed greater intra-individual variability in dementia patients with frontal involvement compared to those with Alzheimer's disease. Increase in intra-individual variability of RT has also been shown to be observed in people with MCI, suggesting the measure as a promising behavioral index that reflects frontal or frontoparietal integrity.