Cognitive control of emotion plays an important role in maintaining emotional stability in people's daily life. However, the neural mechanism remains unclear. This study examined the induced gamma activity in response to emotional expressions which was associated with the cognitive regulation. Electroencephalogram (EEG) was recorded in fifteen normal subjects when detecting emotional expressions. The mean energy was estimated using time-frequency representations in two gamma bands, low gamma band (25-50 Hz) and high gamma band (50-70 Hz), and eight time windows from 0 to 800 ms after the stimulus onset. Two typical gamma activities were observed: (1) the early gamma activity in the 100-200 ms time window was attenuated along with the increased detection difficulty, reflecting the bottom-up attention regulation; (2) the late gamma activity after 400 ms post-stimulus was enhanced with the increased detection difficulty, reflecting the top-down cognitive control. The characteristics of the induced early gamma activity distinguished different mechanisms of attention regulation in the early stage for detecting the negative expression and detecting the positive one. Our study suggested the induced gamma activity was a useful tool to uncover the mechanism of cognitive control of emotion.