Worldwide, breast cancer is the most widespread type of cancer and the second cause of death among women. Mammography is an effective tool for screening women to early detect breast cancer. Aim: Was to determine the effect of instructional module on knowledge, attitudes and practices of women undergoing breast cancer screening mammography. Subjects and method: Design: A quasi experimental research design was used. Setting: Outpatient Gynecological Clinic and Mammogram unit (Comprehensive Clinic of Tanta Main University Hospital), and Mammogram Unit (Tanta International Hospital for University Education) affiliated to Ministry of High Education and Scientific Research, Outpatient Gynecological Clinic (El-Menshawy General Hospital), and Early Detection Unit (Oncology Institute in Tanta) affiliated to Ministry of Health and Population. Subjects: A convenient sample of 100 women (study and control groups) who fulfilled the inclusion criteria. Tools: (I): Women's knowledge regarding breast cancer and mammography, (II): Attitudes (III): Self-efficacy and (IV): Practices regarding mammography. Results: There was a significant relation between knowledge, attitudes, and practices of women undergoing breast cancer screening mammography (p<0.001), as well as between age and self-efficacy (r= 0.346, p= 0.014). There was no relation between knowledge, attitudes, practices, age, education, income, gravidity, and parity (p>0.5). Conclusion: Implementation of the instructional module improved women's knowledge, attitudes, self-efficacy, and practices of women undergoing breast cancer screening mammography. Recommendations: Mammography is a significant technique to early detect breast cancer. It should be made available for all women at all health care settings to decrease mortality and morbidity rates related to breast cancer.