Introduction: Cancer is a type of disorder affecting cell duplication and differentiation which can occur in each part of body tissue at any age, and may cause disease by attacking the healthy tissues of body The present study was conducted in Kerman with the aim of determining the effectiveness of positive thinking training on mental health of the women diagnosed with breast cancer. Methods: The research was a quasi-experimental one with the pre-test and post-test design, having both experimental and control groups. For this reason, 30 patients were selected from among the women diagnosed with breast cancer using purposive sampling method, and then they were put into two (experimental and control) groups, each including 15 patients via random replacement. The positive thinking training course was held during eight 90-minute sessions twice a week for experimental group. Goldberg’s 28-items mental health questionnaire (1972) was used to collect data. Also, the analysis of covariance (ANCOVA) test was used for data analysis with the help of SPSS software (version 19) (p≤0.05). Results: The results obtained from the research showed that the positive thinking training was effective on the mental health of the women diagnosed with breast cancer (P<0.001). Conclusion: Findings of the present research showed that psychological treatments can improve mental pains of cancer patients, Thus, considering the findings of this research, it is suggested to use psychological treatments, which relieve anxiety and depression patients experience during cancer disease پرونده مقاله