This paper focused on longitudinal data analysis of fasting blood sugar (FBS) level of adult diabetic patients at Jimma University Specialized Hospital diabetic clinic using an application of semi-parametric mixed model. The study revealed that the rate of change in FBS level in diabetic patients, due to the clinic interventions, does not continue as a steady pace but changes with time and weight of patients. Furthermore, it clarified associations between FBS level and some characteristics of adult diabetic patients that weight of a diabetes patient has a significant negative effect whereas patient gender, age, type of diabetes and family history of diabetes did not have a significant effect on the change of FBS level. Under various variance structures of subject-specific random effects, the semi-parametric mixed models had better fit than linear mixed model. This was likely due to the localized splines, which captured more variability in FBS level than the linear mixed model.