This study investigated brain functional abnormalities that are specifically related to working memory in amnestic mild cognitive impairment (aMCI) patients using fMRI in combination with an n-back task. Nineteen aMCI patients (aged, 52-78 years, 13 female) and 19 age-and gender-matched healthy adults participated in this experiment. In the task performance, aMCI group had lower accuracy on the 2-back task. The aMCI patients exhibited less areas of activation during performance of the 2-back vs. 0-back tasks, and more deactivation in the default mode network, compared with healthy group. The aMCI patients exhibited decreased activation in dorsolateral prefrontal cortex and supplementary motor cortex during the 2-back vs. 0-back tasks, compared with healthy group.