We studied the mechanism of electroencephalography (EEG) in the elderly with mild cognitive impairment (MCI) by analyzing the Lempel-Ziv complexity (LZC) of EEG during a cognitive task. We recorded the EEG simultaneously when they were asked to judge whether the color of two graphics was matched or not. The results showed that the LZC of EEG in the MCI group was significantly higher than those in the normal control group early in the period of cognitive activities (P < 0.05). The LZC in MCI patients decreased gradually over time, while results of the normal group kept unchanged. There was no significant difference in LZC between two types of tasks. The two groups showed similar regularity in spatial distribution.