Bachground: Previous studies have shown that Unified transdiagnostic therapy affects different variables. However, there is a research gap regarding the effectiveness of Unified transdiagnostic therapy on the Positive Metacognitions and Meta-Emotions in mothers of children with cancer. Amis: The purpose of this study was to determine the effectiveness of Unified transdiagnostic therapy on Positive Metacognitions and Meta-Emotions in mothers of children with cancer. Method: This research was a quasi-experimental study with pre-test and post-test with control group.The statistical population consisted of all mothers of children with cancer who were referred to Mahak, Shohada Tajrish, Bahrami, Hazrate Ali Asghar hospitals, in 2019 in Tehran. 31 mothers were selected by available sampling method and randomly assigned to the experimental group and the control group. The present research tools are Positive Metacognitions and Positive Meta-Emotions Questionnaire (Beer,2011) and Unified Transdiagnostic Therapy (Barlow, 2011). The experimental group received the Unified Transdiagnostic Therapy for 12 sessions, each for 90 minutes.Data analysis was performed by covariance analysis method. Results: Unified transdiagnostic therapy on improving the components of confidence in extiguishing perseverative thought and emotions, confidence in interpreting own emotions as cues, restraining from immediate reaction and mind-setting for problem-solving and confidence in setting flexible and feasible hierarchies of goals had a significant effect (P≤ 0/001). Conclusions: Mothers who participated in Unified transdiagnostic therapy sessions had significant improvements in Positive Metacognitions and Meta-Emotions.