Interest in microcantilever based biosensors in the biomedical field has largely increased during the last years. Potentially, this kind of sensor can provide a considerable contribution to complex disease diagnosis, which requires the detection of biological molecules. Microcantilever biosensors allow the detection of complementary DNA fragment hybridization or specific antibody-antigen binding; it is known that adsorption of specific biological molecules upon the microcantilever surface induces cantilever deflection due to the interaction of the molecules with the surface. To date, the phenomena which determine the deflection mechanism are not completely known. The present work investigates the electrostatic field within the molecules and the forces consequently acting on the molecules and on the cantilever in order to provide a description of the deflection mechanism. The electrostatic potential of arrays of double strand DNA molecules immersed in an ionic solution was modelled by means of cylinders negatively charged at the surface and a FE (Finite Element) continuum electrostatics analysis was implemented in order to numerically solve the second order non-linear Poisson-Boltzmann equation. Then, a FE structural analysis of the cantilever was performed coupled with the continuum electrostatics analysis. In this way, the effects of the molecules’ electrostatic interactions on the cantilever deflection were taken into account. The model was run to describe the microcantilever deflection due to the electrostatic field under different design and operating conditions, and it was also set to compare hexagonal and square disposition of double strand DNA molecules.