Using fMRI, neural substrates of verbal working memory were investigated with respect to differences in working memory capacity. Listening-span test (LST), Listen, and Remember conditions were performed. Two subjects groups were selected: those who had large working memory capacities, labeled high-span subjects (HSS) according to the working memory span test, and those who had small working memory capacities, labeled low-span subjects (LSS). Significant activation was found mainly in three regions in comparison with resting control: left prefrontal cortex (PFC), anterior cingulate cortex (ACC) and temporal language area. For both groups, fMRI signal intensity increased in PFC during the LST condition compared to the Listen condition. A group difference was found in the ACC region; specifically, a significant increase in signal intensity was observed in ACC only for the HSS group and not for the LSS group. Behavioral data also showed that the performance was better in HSS than in LSS. These results indicate that the attention controlling system, supported by ACC, is more effective in HSS compared to that of LSS.