Purpose: The purpose of this study was to identify the level of psychological insulin resistance and self-care activities and to evaluate the factors affecting self-care activities in patients with type 2 diabetes mellitus undergoing insulin therapy to provide basic data for the development of educational programs. Methods: Data were collected through the interviews using a structured questionnaire from August 29 to October 20, 2017, from the patients with type 2 diabetes mellitus visiting the Diabetes Mellitus Center at H-General Hospital in J-city. The subjects were 168 patients who had been being treated via self-injection for at least three months after the start of insulin therapy. Data analyses were conducted using t-test, ANOVA, Pearson’s correlation coefficients, and stepwise multiple regression using the SAS WIN 9.2 program. Results: The mean score of psychological insulin resistance was 61.25 (range 19-95) and the mean score of self-care activities was 53.19 (range 18-90). Self-care activities were significantly different by gender (t=-2.94, p=.004), perceived health status (F=7.00, p<.001), and hypoglycemia during the last three months (t=-2.47, p=.015). Negative correlation was observed between psychological insulin resistance and self-care activities (r=-.33, p<.001). Self-care activities were significantly predicted by psychological insulin resistance, perceived health status, gender, and hypoglycemia during the last three months, and 19.0% of the variance in self-care activities was explained (F=9.01, p<.001). Conclusion: Psychological insulin resistance in patients undergoing insulin therapy and its effects on self-care activities identified in this study will be useful in starting and maintaining insulin therapy in the future.