Purpose:The purpose of this study was to investigate diabetic knowledge, self-care behaviors, and HbA1c of patients with diabetic mellitus in public hospitals. Methods: Participants were 287 adult patients who had been diagnosed with diabetes in two public hospitals in Seoul. Collected data were analyzed using t-test, ANOVA, Pearson's correlation with the SPSS (PASW) 21.0 program. Results: For general characteristics of the participants, there were significant differences in the level of diabetic knowledge according to age (F=11.28, p<.001), educational level (F=11.07, p<.001), type of health insurance (F=9.38, p<.001), and monthly income (t=-4.58, p<.001) and in the self-care behaviors according to age (F=8.61, p<.001) and in HbA1c according to age (F=6.72, p=.001). As for disease related characteristics of the participants, significant differences were found for self-care behaviors according to education about diabetes (t=3.90, p<.001) and in HbA1c according to education about diabetes (t=3.26, p=.001) and current diabetic therapy methods (F=13.51, p<.001). The study results showed that there was no correlation between diabetic knowledge and self-care behavior, or between self-care behavior and HbA1c. Conclusion: Results indicate that when developing preliminary data on nursing intervention education programs on diabetes, increasing diabetic knowledge and self-care behaviors would help to improve blood glucose levels.