Purpose: The purpose of this study was to investigate the level of knowledge, health belief, and self-efficacy affecting on breast self-examination (BSE), and to identify factors influencing the practice of BSE of women in their 30`s and 40`s. Methods: With a correlation survey design, 194 women in their 30`s and 40`s were recruited at 4 culture centers in Busan via convenience sampling. Measures were included with knowledge, health belief, and self-efficacy for breast self-examination and level of BSE practice. Results: Level of knowledge, health belief and self-efficacy for BSE were greater than medium level. BSE practice had positive correlations with subscales of health belief, except barrier and self-efficacy, but no relationship with knowledge. Self-efficacy, sensitivity, and benefit among the entered variables were significant factors influencing the practice of BSE, and explanatory power of these variables was 32.8%. Conclusion: Based on this study, studies are required to compare through repeated research according to age, education, occupation, environmental characteristics. In addition, to maximize the educational effect, development of the educational program in conjunction with the local community to increase breast self-examination practice and research about the effectiveness are needed.