The purpose of this study was to explore the relationships among illness knowledge, social support and self-care behavior in adolescents with beta-thalassemia major. The subjects were 58 beta-thalassemia major adolescents recruited from the pediatric hematology outpatient departments of three hospitals in North Taiwan. All data were analyzed by descriptive statistics, one-way ANOVA, t-test, Pearson correlation, and stepwise multiple regression. The results showed that: (1) Scoring in illness knowledge, the best was treatment knowledge, and the worst was the knowledge of complications and symptoms. (2) The result for social support indicated that the family was the major source of support, and that classmates/friends provided the least support for thalassemic adolescents. (3) Scoring in self care behavior, the best was the medical and chelate therapy, and the worst was the management of problems and coping. (4) Individual characteristic differences, including sex, level of education, length of illness, complications of thalassemia, hospitalization due to thalassemic complications and receiving other treatments were correlated with their illness knowledge, social support or self-care behavior. (5) Illness knowledge, social support which were positively correlated with self-care behavior. (6) 50.0% of the variance in self-care behavior can be explained by emotional support from family, general knowledge of thalassemia, treatment knowledge and appraisal support from family. These findings could provide referential material for nursing research and nursing practice.