Background: Diabetes Mellitus (DM) is a chronic, progressive disease characterized by elevated levels of blood glucose. Despite the fact that most international association/organization gave attention toward diabetes control and prevention by healthy professional, still diabetes and its complication such as cardio vascular, blood vessels, eyes, kidneys and nerves become a major cause of premature death and disability across the world. The overall aim of this study is assessing fasting blood sugar variation over time and its determinant among diabetic patients. Methods: Data were obtained from Adama Hospital Medical College diabetic patients who have been active in the follow-up treatment from September 1, 2018 to August 30, 2019. The data consists of basic demographic and clinical characteristics of 312 DM patients were selected using simple random sampling techniques and of whom 177 were males and the rest 135 were females. The linear mixed effect model for longitudinal data analysis was used by taking the correlation between Fasting Blood Sugar (FBS) level of patients into account. Linear mixed model, random intercept and slope models were used for feting the data. Results: The results from the linear mixed model with unstructured co-variance structure showed that for one-month change in time decreases log FBS level by 0.0111267 mg/dl. While a unit increase in Body Mass Index (BMI) of a patient on treatment, the log FBS level was increased by 0.0434 mg/dl. Similarly, a unit increase in Diastolic Blood Pressure (DBP), the log FBS level was increased by 0.0004749 mg/dl. Being tertiary and secondary level of education decreases logFBS level by 0.0058844 and 0.0055161 respectively compared with patients with no education.Conclusion: Age, Educational status, Dietary type, Drug type, History of hypertension, BMI, DBP, Time, interaction effect of Age, history of hypertension, Dietary type, other comorbidity at baseline with time were the significant determinant for the change in mean FBS level of the diabetes patients over time. Based on the findings of our study and WHO recommendation, maintaining of healthy body weight, by taking healthy diet along with lower blood glucose level is essential to control blood sugar in body and to prevent long term complication.