Abstract: An objective and feasible index for monitoring the mental fatigue degree was proposed.A mental fatigue model was established.There spectrum-related features including dominant frequency,barycenter frequency and relative power in different rhythm were extracted from the electroencephalogram before and after fatigue.The relationships between such three features and brain fatigue states were analyzed,and then did statistical analyses of experimental data.The results show that the EEG power moved to low frequency segment,the dominant frequencies and barycenter frequencies of all electrodes decrease,the relative power in θ rhythm increases,but the relative power in β rhythm decreases when the mental fatigue degree increases.Therefore,the spectrum-related features of EEG can reflect the degree of mental fatigue,and may hopefully become the objective index for monitoring the mental fatigue degree.