Background: Diabetes mellitus (DM) is a chronic progressive metabolic disorder characterized by hyperglycemia mainly due to absolute (Type 1 DM) or relative (Type 2 DM) deficiency of insulin hormone. Diabetes Mellitus is a complex chronic disease, requiring continuous medical care with multi factorial risk reduction strategies beyond glycemic control. Objective: The present study aims to determine the role of the Trans-theoretical Model in enhancing glucose level - controlling behavior for clients with DM. Patients and methods: Simple random sample of 60 diabetic clients who are attending Primary Healthcare Centers at Al-Karkh District of Baghdad were included in the present study. The participants were divided into experimental group (N=30) and control group (N=30), matched for age and gender. Results: The mean of age of participants was 55.7-year-old, and the majority was males. First degree relatives with Type 2 DM were within positive bio-social aspect. Laboratory screening had a direct impact on the incidence of Type 2 DM. There was highly significant comparative difference between the pretest and posttest I of the experimental group relative to glucose level controlling behavior. Also, there is highly significant comparative difference between the posttest I and posttest II of the experimental group relative to glucose level controlling behavior. In addition, there was highly significant difference between pretest, posttest I and posttest II of the experimental group relative to glucose level controlling behavior. Conclusion: Trans-theoretical Model for Change is effective in enhancing glucose level controlling behavior for clients with DM.