Purpose: This study aims to determine obstetrics nurses knowledge, attitude, and practice about breast cancer and breast self-examination (BSE), and to contribute to the early detection of breast cancer during breastfeeding periods. Methods: For the survey, 163 individuals (obstetric nurses) were chosen by convenience sampling and agreed to participate in the study. Their knowledge, attitude, and practice about breast cancer and BSE. Results: Knowledge and attitude averaged $70.45{pm}10.90$ (of 100) and $3.64{pm}0.27$ (of 5), respectively. While most nurses (99%) recognized the importance of BSE, only 58.9% experienced BSE. BSE practice level averaged $8.35{pm}1.96$ (of 12). Only 20.2% had recommended BSE to their clients. Practice level varied significantly for different marital status, breastfeeding experience, and education, while knowledge and attitude remained independent. Nurses who had experienced mammogram or breast ultrasonogram themselves scored higher in knowledge. Attitude was higher for nurses who received recommendation for BSE, performed BSE, received BSE education, or recommended BSE to clients. Practice level was higher for nurses who received BSE education or willing to perform BSE in future. Practice level had a positive correlation with attitude but no correlation to knowledge. Conclusion: Obstetric nurses need continuing education for practicing BSE. Practical BSE education can not only promote preventive behavior of nurses, but it can also improve the breast health management of obstetrical clients.