In this study a general idea about the biosensor was represented, with a particular reference to diabetic biosensor. In the experimental part of this work a comparative investigation was carried out for the estimation of the blood glucose, by using two types of measurements. The first method was based on measuring the glucose with a commercial biosensor (Glucotrend2); whereas, in the second method a biochemical assay was used. 40 diabetic patients and 55 normal control subjects were taken, and it was found that there was a significant difference between the results of blood glucose estimation between these two methods, in spite of using the same sample of blood. This significant difference was seen in all age groups, in both male and female and also was seen in control and diabetic subjects. The possible factors causing these differences were discussed and an equation was presented to correlate the results of one method to the results of the other. The theoretical part of this study involved producing two biosensors model with their electrical representation. The first was based on using a bioluminescence component such as luciferin or one of its derivatives which emit light when reacting with any of glucose-glucose oxidase (G.O.D) reaction components. The second model was based on measuring the difference in the intensity of light that enters the sample and the light coming out from it; in this design the results of reaction between glucose and G.O.D in the presence of Chromogen were considered for measurement with light by changing their color or by changing their transparency.The two models were simulated using MATLAB and electrically proposed using OrCAD. The parameters that will affect the response of the reaction and those controlling the model were changed to obtain the response to the biological component. The change in the biosensors parameters will affect the sensitivity and the efficiency of the biosensor, while the parameters that control the bioreceptor will affect the response time to reach a steady state as, well as , the sensitivity of the biosensor. This study can be considered as the first step for the future to understand the biosensor and there application in life.