Abstract: This study is aimed to research the effects of mental fatigue caused by long-term mental activity on brain functional connectivity,and investigate objective indicators of mental fatigue evaluation.Mental fatigue was induced by a sustained cognitive task.Using mutual correlation in time domain,the correlation properties of electroencephalogram (EEG) data recorded in two states (normal state and mental fatigue state) were analyzed.The brain functional networks were constructed.The characteristic parameters were statistically analyzed based on the complex network theory.The results showed that the scales of subjective perceived fatigue were increased after the sustained cognitive task.The average degree,average clustering coefficient and network density were significantly decreased during mental fatigue,while average path length was significantly increased.The characteristic parameters of brain functional network could reflect changes of brain functional connectivity during mental fatigue.