Microcantilever based biosensors are increasingly used for several bio-applications. In this paper a model based on electrostatic interactions between bio-molecules is proposed to explain the cantilever deflection in biosensor. This modeling, at molecular level, is difficult due to large number of molecules and large simulation times involved. This problem is overcome by evaluating the electrostatic interactions between the groups of molecules rather than the individual molecules. The analysis shows that for sufficiently fine grids (small groups of molecules), the results don't vary much even if the grid is made finer. Simulations carried out verify the analytical results. Simulations carried out also show that the cantilever biosensor deflections match fairly with the experimental data obtained for myoglobin marker for myocardial infarction (MI) published recently. The discrepancies are attributed to surface irregularities which are not considered in the model. It is also observed that the deflection obtained for non-uniform distribution of biomolecules on the cantilever surface (more close to the actual case) doesn't vary much from a uniform distribution. Though the model is validated using results for myoglobin markers used in detection of MI, the modeling and simulation methodology is fairly general and will be valid for other biosensors.