Objective: The aim of this study was to investigate the effect of case management on glycemic control glycosylated hemoglobin (HbA1c) of older people with type 2 diabetes mellitus. Methods: This study used the quasi-experimental one-group pre-post test designed to measure the effect of the intervention using glycemic control in glycosylated hemoglobin (HbA1c) of older people with type 2 diabetes mellitus. Thirty patients in outpatient department selected by means of purposive, qualification-based sampling were given a case management program based on the National Case Management Task Force, the CMSA's Board of Directors (2009) including comprehensive health assessment, a clinical pathway developed by a multidisciplinary team, health education and consultation with nurse case manager of diabetes mellitus and hypertension, a home visits, follow-up telephone calls and individual learning activities. Glycemic control in glycosylated hemoglobin (HbA1c) tests was collected before and after the program in the twenty-fourth weeks. Descriptive statistics and paired t-tests were used to analyze the data. Results: After the twenty-fourth weeks, it was found that glycemic control in glycosylated hemoglobin(HbA1c) test decreased significantly (p-value < .05). The study findings support the effect of the case management program. Conclusion: The findings of this study proved that the case management program is an essential component in the management of older people with type 2 diabetes mellitus. Application of a case management program based on CMSA (2009) model for diabetes mellitus prevention in outpatient department setting is therefore recommended. In addition, the older people with type 2 diabetes mellitus enrolled in the case management program with nurse case manager have responded in physical, mental, emotional, and social contact and home visits by multidisciplinary team. The older people with type 2 diabetes mellitus were should be taken care of and get supported from their beloved family. The case management program should be applied for other chronic disease.