This paper presents method for modeling fluctuation of blood sugar in diabetic rats. 32 male wistar rats are randomly divided in to control, control+extract, diabetic and diabetic +extract groups ( n=8). Diabetes is induced by intraperitional injection of 60 mg/kg b.w of streptozocin (STZ). Treatment with aqueous-ethanolic extract of cinnamon (80 mg /rat/ day) is followed for 8 weeks and blood sugar is measured weekly. The measured data are used as examples and fuzzy model was obtained. The proposed method can model the blood sugar fluctuation with good fitness and it can be used for blood sugar fluctuation prediction. Finally the proposed method is simulated and the simulation results are very promising.