Fractal dimension and Lyapunov exponent analysis of digital EEG data was performed to compare differences among various mental activities and for determining the significance of chaotic patterns. Different kinds of tasks were performed while EEG recordings were being made. These tasks including opening and closing eyelids, calculation, listening to music, and remembering pictures. For more than one minute, signals of each performed task were recorded as a digital EEG. CHASIM (our own software) was used to calculate the fractal dimension and Lyapunov exponent. Statistical analysis was performed in GLM repeated measures using SPSS 10.0 for Windows. Fractal dimensions were high during the activities of "listening to music and noise" and "moving both toes". Fractal dimensions were low during "eye opening" and "memorizing the pictures" activities. We suggest the auditory stimulation gave more widespread brain activities than other stimulation including calculation and visual stimulation.