This study aimed to compare the effectiveness of the mother's mindfulness-based cognitive therapy and cognitive emotion regulation training on externalizing disorder and selfefficacy of aggressive children. Methods: The research method was a quasi-experimental study with a pre-test/post-test design and a control group. The study population (N=540) included all female students in the primary schools in Namin City, Iran in 2018 with their mothers. A multistage clustering random sampling was used to choose the students under study, and a total of 217 students were selected as the samples and the aggression questionnaire was completed by the teachers. Of these students, 79 students who scored high on the Shahim's aggression scale were selected as aggressive students and 60 mothers of these students were selected by considering the inclusion criteria and were assigned randomly in three groups of 20 mothers each. The mindfulness-based cognitive therapy and cognitive emotion regulation training were executed in the experimental groups in 8 sessions, whereas the control group received no intervention. Before and after the intervention, all three groups responded to Shahim's relational aggression scale, Sherer and Adam's general self-efficacy, and Achenbach and Rescolar's child behavior checklist. The obtained data were analyzed by multivariate analysis of covariance. Results: The calculated values showed that mindfulness-based cognitive therapy was more effective (P>0.05) than cognitive emotion regulation training. Conclusion: the findings of thi s study suggest that mother's mindfulness-based cognitive therapy can be an effective intervention in reducing externalizing disorders and increasing self-efficacy in aggressive children.