The present study was conducted to investigate The effectiveness of mindfulness based stress reduction training on the cognitive fustion and alexitimia of adolescents with internalizing behavior disorder. The present study was semi experimental (with experimental and control groups) with three-stage (pretest, posttest, follow-up) and two-month follow-up period design. The statistical population of the research included adolescents with internalized behavior disorder who were studying in the first period of secondary school in the academic year of 2022-2023 in Qom city. 37 adolescents with internalizing behavior disorder were selected through purposive sampling method and randomly accommodated into experimental (19 people) and control groups (17 people). The experimental group received ten ninty-minute sessions of mindfulness based stress reduction training during ten weeks. The applied questionnaires included Cognitive Fustion Questionnaire (Gillanders et al, 2014), Alexitimia Questionnaire (Bagby, Parker, Taylor, 1994) and Children and Adolescents Behavioral Inventory (Achenbach, Rescorla, 2001) (CABI). The data from the study were analyzed through mixed ANOVA method via SPSS23 software. The results showed that mindfulness based stress reduction training has significant effect on the cognitive fustion and (F=39.15; Eta=0.53; P<0001) and alexitimia (F=73.07; Eta=0. 68; P<0001) of adolescents with internalizing behavior disorder. According to the findings of the present study it can be concluded that mindfulness based stress reduction training can be used as an efficient method to decreace cognitive fustion and alexitimia of adolescents with internalizing behavior disorder through applying the how to deal with stress and act mindfully (breathing, eating, sitting and walking mindfully).