Diabetes mellitus is a chronic disease that requires lifelong self-management. The author used a transtheoretical model to help improve the self-care abilities of one diabetes mellitus patient who lacked initial appreciation / understanding of self-care needs. During hospitalization, the author employed observation and communication to evaluate subject needs with regard to transforming her dietary behavior in order to better stabilize her medical condition. The author educated the subject on a one-to-one basis in dietary control using change tactics, and recommended that she adjust her 12 portions and food exchange. The patient was instructed to operate the blood sugar machine to monitor the effects of diet control. This method was designed to help the patient build self-care decision-making abilities during the action phase and bolster confidence during the maintenance phase through self-efficacy promotion, and to increase confidence in her self-care abilities. Telephone interview follow-ups demonstrated that the patient was able to self-comply with 12 portion rules and monitor her blood sugar on a daily basis. With blood sugar measurements at 70－105mg/dl and 110－160mg/dl, respectively, before and after meals, her diet control remained in the action phase. The patient was able to self-monitor her personal health status via blood sugar level and conduct personal healthcare maintenance work. Results support the effectiveness of applying the transtheoretical model to self-care strategies for diabetic patients.