Coplanar capacitive technique is a relatively novel electro-magnetic Non-Destructive Testing (NDT) method that could be applied to the evaluation of materials by moving a set of electrodes on the surface of the specimen. In addition to the design-related parameters such as electrode shape, size, and the separation distance between the main electrodes, the material of the specimen affects the coplanar capacitive probe performance. In this paper, a 3D Finite Element Modeling (FEM) was employed to assess and identify the electric field behaviour as a function of material under test for non-conducting and conducting specimens with/without defect. Physical experiments were carried out by a pair of rectangular coplanar electrodes on an aluminium specimen with surface defects covered by a 5 mm thick plexiglass insulation layer to verify the simulation results and evaluate the performance of the probe. A good qualitative agreement was observed between the numerical simulations and experimental results.