A theoretical study on a novel planar waveguide surface Plasmon Biosensor is presented in this paper. The proposed biosensor has a configuration similar to the Otto excitation mechanism for surface Plasmon polaritons. The performance of the device with respect to key system parameters such as gap-width and device length is investigated using an eigenmode solver with perfectly matched layers (PML). Device resolution of 2.3 x 10(-6) RIU has been demonstrated for an aqueous analyte.