The study of non-linear EEG is of great significance in clinical practice and research work. This paper has gone into the feasibility of calculating the correlation dimension and has developed some subjects with the characters of correlation dimension and the difference under four conditions: (1) passive eyes closed(PEC); (2) mental arithmetic with eyes closed(MAEC); (3) passive eyes open(PEO); (4) mental reasoning with eyes open (MRED). The results show it is feasible and meaningful to calculate correlation dimension and the correlation dimension can reflect the regular patterns of mental activity.