Objective To explore the activity and its possible neural mechanism of brain default mode network by using resting state functional magnetic resonance imaging (fMRI) in patients with mild cognitive impairment (MCI). Methods The 20 amnestic MCI patients and 25 healthy controls were included in this study, and all subjects underwent mini-mental state examination (MMSE), auditory verbal learning test (AVLT) and fMRI. The data were analyzed by amplitude of low frequency fluctuation (ALFF), and the enhanced and weakened regions of ALFF were observed and compared in both MCI patients and healthy controls. Results MMSE and AVLT tests showed that the memory function was seriously impaired in MCI patients compared with healthy controls, which is based on the short and long delayed episodic memory impairment (2.4±1.7 vs. 6.6±1.4, t=3.70, P＜0.01; 2.1±1.6 vs. 6.7±1.5, t=4.16, P＜0.01). The resting state fMRI showed that MCI patients had significant decreases of ALFF in hippocampal formation, parahippocampal cortex and lateral temporal cortex as compared with health controls (t=2.58, 2.43 and 1.75, all P＜0.01), which were closely relevant to the episodic memory. And they had significant increases in temporal-parietal joint and inferior parietal lobule (t=3.14 and 2.77, both P＜0.01). Conclusions MCI patients show significant decreased active intensity of some DMN nodes that is related to episodic memory in resting state. Increased active intensity in MCI patients would be some type of compensation.