Purpose: The goal of this study was to find out factors influencing the health promotion behavior of low-income vulnerable 4th, 5th and 6th-grade elementary school students. The specific goals were: first, to find out difference in health knowledge, self-esteem and health promotion behavior according to general characteristics; second, to investigate the correlations among health knowledge, self-esteem and health promotion: and, third, to analyze factors influencing health promotion behavior. Methods: The subjects of this study were 137 low-income vulnerable 4th, 5th and 6th-grade elementary school children who were participating after-school programs in Seoul. Results: Statistically significant differences were observed in health knowledge, self-esteem and health promotion behavior between girls and boys. In the sub categories, differences were observed in personal hygiene and health responsibility, stress management and personal relationship. The correlation of health promotion behavior with self-esteem and health knowledge was statistically significant. Regression analysis revealed that the influencing factor is self-esteem with the other variables under control. Conclusion: Health promotion education requires low-income vulnerable elementary children to increase their self-esteem. We recommend that it should be one of the most effective ways to split boys and girls to educate them in disparate classrooms.