The present study examines how gamma oscillations can be modulated by cognitive reappraisal. Thirty-two participants performed cognitive reappraisal tasks to increase or decrease their emotions while viewing neutral and emotional (positive or negative) pictures. As a control task, the participants also simply viewed the pictures without an attempt to manipulate emotions. Electroencephalograms of the participants were recorded during the cognitive reappraisal tasks to extract the gamma oscillations. Gamma activity was quantified by measuring event-related changes in the gamma band power (30–55 Hz). During the tasks, the greatest gamma activity was found at the parietal regions. Greater parietal gamma activity was induced by the emotional pictures compared with the neutral ones when the participants passively viewed the pictures. No such difference in the parietal gamma activity was found when the participants performed the cognitive reappraisal tasks. In addition, parietal gamma activity during the cognitive reappraisal tasks was greater than that during the control task in response to the neutral pictures, implying the role of parietal gamma activity in the top-down cognitive execution process. In contrast, parietal gamma activity during the cognitive reappraisal task to increase emotions was greater than the levels of activity during the task to decrease emotions and during the control task in response to the emotional pictures, implying the role of parietal gamma activity in the bottom-up sensory emotional process. These results suggest that parietal gamma activity may be implicated in the multiple cognitive reappraisal processes.