The study was conducted to determine the effect of mindfulness training on cognitive flexibility in sixth-grade female students. This research used a pretest-posttest study with control group. A multi-stage random sampling was used in 4 schools, among which 37 sixth-grade female students were randomly selected and assigned to two experimental (17 students) and control groups (20 students). Students in the experimental group participated in a mindfulness training course for 12 (120-minute) sessions for a month and a half. The study used Dennis and Vander Wal's Cognitive Flexibility Inventory (2010). The results of covariance analysis showed that there was a significant difference between the two groups in the scores of cognitive flexibility and due to the higher mean scores of the cognitive flexibility of the experimental group in the posttest (p> 0.05, p = 10.25), it can be stated that mindfulness training was effective in increasing students' cognitive flexibility.