Objective The purpose of this study was to investigate functional connectivity (FC) changes in the default mode network (DMN) in untreated patients with first episode of major depressive disorder (MDD) by using restingstate function magnetic resonance imaging (rs-fMRI) and the seedbased analysis method. Methods 16 MDD patients and 19 healthy controls with matched age, sex and education were included and underwent rs-fMRI. Functional connectivity was conducted between the time courses of the posterior cingulate cortex and each voxel. Significantly changed (P＜0.01) brain region which was related to DMN network was selected as region of interest (ROI). Further linear correlation analysis was performed between the functional connectivity coefficient and Hamilton depression scale score. Results The brain regions of DMN in the MDD patients distributed in bilateral precuneus, left medial and parietal cingulate gyrus, right medial frontal gyrus, right medial frontal gyrus and right superior temporal gyrus. The brain regions of DMN in healthy controls distributed in bilateral precuneus, right angular gyrus, bilateral medial frontal gyrus, and bilateral middle temporal gyrus. Compared with healthy controls, the DMN connection strength of left medial frontal gyrus was significantly decreased in the MDD patients (P＜0.01). There was no correlation with difference between the connection strength of left medial frontal in the MDD patients and the Hamilton Depression Scale (r=0.084, P=0.756), the duration of MDD (r=0.116, P=0.670). Conclusion There is abnormal function of DMN in untreated patients with first episode of MDD, but it is not found to be related to the severity of depression and the duration of seizures.