Objective. To establish method in analysing EEG complexity as related to brain function state (Cs) and to examine Cs under conditions of taking rest and doing mental arithmetic, to explore their regular changes. Method. EEG of 10 male subjects aged 26-30 under the conditions of taking rest and doing mental arithmetic were recorded. Cs under the two conditions and their changes related to brain function states were analyzed with a special software for Cs analysis. Result. EEG Cs under mental arithmetic changed significantly. According to these changes, the subjects were divided into two groups: Group A, 8 subjects, their EEG Cs under mental arithmetic were significantly higher than Cs under rest at all electrode sites. The most outstanding areas were temporal region and parieto-occipital region. Group B, 2 subjects, their EEG Cs under mental arithmetic were significantly lower than Cs under rest. The most outstanding areas were prefrontal region and temporal region. Conclusion. The result showed that high correlation existed between mental arithmetic and Cs. This method of analysis was useful in studying brain cognitive activities and assessing mental workload.