Background: Anxiety among patients after surgery can affect their physiological and psychological well-being. The aim of this study was to investigate and compare the effects of Benson’s relaxation and rhythmic breathing techniques on postoperative anxiety in candidates for the mastectomy surgery. Methods: This randomized controlled clinical trial study was conducted with ninety patients in 2013. The patients were hospitalized for the mastectomy surgery in three surgical wards in a teaching hospital, Tehran, Iran. They were randomly assigned into three groups: Benson’s relaxation including the cognitive relaxation technique type, rhythmic breathing including the somatic relaxation technique type and control groups. According to the Davidson and Schwartz multi-process theory, the Benson’s relaxation and the rhythmic breathing techniques have cognitive and somatic effects, respectively. One day before the surgery, the patients in the intervention groups were trained regarding relaxation and breathing techniques and were asked to perform the techniques under the supervision of the researcher in the night before the surgery. The cognitive somatic anxiety questionnaire was used to measure anxiety before the intervention and half an hour after recovery of consciousness after the surgery. Descriptive and inferential statistics were used for data analysis via the SPSS v.21 software. Results: There were no statistically significant differences between the groups in terms of demographic characteristics. The application of both techniques reduced the level of patients’ anxiety after the surgery. The patients in the Benson’s relaxation technique group reported only the relief of somatic anxiety. However, the breathing technique patients reported a reduction in both cognitive and somatic anxiety. Conclusion: The Benson’s relaxation and rhythmic breathing techniques can reduce postoperative anxiety in patients after the mastectomy surgery. Trial Registration Number: IRCT2014042017350N1