Waveguide-based surface plasmon resonance (SPR) sensors are analyzed using both 2-D and 3-D beam-propagation methods and the sensing characteristics are discussed for aqueous environment. First, the effect of the metal thickness on the sensing characteristics is investigated. It is shown that the maximum absorption wavelength shifts to a longer wavelength, as the metal becomes thick. Next consideration is given to the SPR sensor with the adsorbed layer placed on the metal layer. The change in the adsorbed layer thickness also leads to the shift of the maximum absorption wavelength. The reason for the wavelength shift is explained in detail using the eigenmode analysis. The configuration parameters are determined for the sensor operation around a wavelength of 0.6 μm. As a result, the sensor shows the absorption wavelength shift from 0.594 to 0.602 μm, when the refractive index of an analyte is increased from 1.330 to 1.334. Finally, the numerical results of the 2-D model are compared with those of the 3-D model.