Background: Cancer diagnosis for everybody may be perceived as crisis and breast cancer, as the most common malignancy in women, can influence their well- being and multiple aspects of their health. So understanding that how women in various contexts and communities adjust to the illness is necessary to facilitate this adjustment and improve their quality of life. Objectives: The aim of this study was to: 1) identify the core components of coping strategies to adjust to the illness in Iranian women with breast cancer perspective, 2) to develop and determine psychometric properties of a native self- report instrument to assess coping behaviors and measure the degree ofadjustment with the breast cancer. Methods: The present exploratory mixed method study was conducted in two consecutive stages: 1) the hermeneutic phenomenological study was done to explore the life experiences of coping styles to adjust with the breast cancer using in-depth interviews with patients that lead to item generation; 2) psychometric properties (validity and reliability) of the instrument were evaluated recruiting 340 eligible women. The item pool was reduced systematically and resulted in a 49-item instrument. Results: From the qualitative stage, item pool containing 78 items related to coping strategies to adjust with the breast cancer. After eliminating unwanted statements from the results, qualitative and quantitative face and content validity, the 10 factors extracted employing construct validity were: feeling of guilt, abstentiondiversion, role preservation and seeking support, efforts for threat control, confronting, fear and anxiety, role wasting, maturation and growth, isolation, and fatalism. These factors accounted for the 59.1% of variance observed. The Cronbach reliability test was carried out and alpha value of 10 factors was calculatedfrom 0.78 to 0.87 confirming all factors were internally consistent. The scale s stability was tested using the test-reset method. Conclusion: The 49-item AIMI-IBC revealed acceptable psychometric properties. This instrument provides healthcare professionals to systematically assess the coping strategies of Iranian women with breast cancer and measure the degree of adjustment with illness.