Background and Aim: The role of emotion regulation in mental illness and learning and functional abilities of adolescent students has been confirmed today and is an important basis for the emergence of emotional and behavioral disorders during adolescence. Accordingly, this study was conducted to determine the effectiveness of mindfulnessbased group therapy (MBGT) on cognitive emotion regulation (CER) in female adolescents. . Methods: The study had a pretest-posttest design with a control group and a two-month follow-up. The statistical population included all high school students in Saveh city (Markazi province) in the academic year 2016-2017, from which 30 people (15 in the experimental group and 15 in the control group) were selected by cluster sampling and randomly assigned to experimental and control groups. The study tool was the Cognitive Emotion Regulation Questionnaire (CERQ) developed by Granefski et al. (2001). The experimental group received eight sessions of mindfulness training, and the control group received no intervention. Data were analyzed using repeated-measures ANOVA. Results: The results suggested that MBGT affected CER in female adolescents and that this effect was stable in the follow-up phase. The results also indicated that MBGT increased positive CER and decreased negative CER (P = 0. 01). Conclusion: According to the findings, counselors and psychologists are recommended to use MBGT as an effective method to increase positive CER in female adolescents.