Objectives: The present research has been carried out to investigate the effectiveness of group mindfulness-based cognitive therapy on modification of body image and quality of life in women with breast cancer undergoing mastectomy. 
Materials: This research is a quasi-experimental study (pretest-posttest with control). In this clinical trial, 50 voluntary patients who had undergone mastectomy in 2013 and 2014 were screened among the patients admitted to Omid Hospital and Reza Treatment Center in Mashhad and were randomly divided into experimental group (n=25) and control group (n=25). First, both groups were pretested. Then, the experimental group received group mindfulness-based cognitive therapy treatments during eight sessions of 120 minutes (one session per week), but the control group received no treatment. At the end of treatment, both groups received the post-test. Data collection tools included standard questionnaires EROTIC QLQ-Br 23.V.3 and the quality of life questionnaire (SF-36). The data were analyzed based on covariance analysis using SPSS software (Version 20).
Results: Comparison of the mean scores of quality of life and body image showed a significant difference between the control and experimental groups after receiving the treatment (P<0.01). In other words, mindfulness-based cognitive therapy was effective on the quality of life and modification of body image in patients. 
Discussion: Group mindfulness-based cognitive therapy can be considered as an effective and efficient approach to the quality of life and modification of body image in women with breast cancer who have undergone mastectomy.