The resonance properties in a guided mode resonance (GMR) structure having a cladding layer inserted between the subwavelength grating and the high-index film are studied. The presence of the layer results in a significant reduction of the resonance linewidth where multiple-coupling approximations are implemented to describe this behavior. The sensitivity of the resonance wavelength to the change of the layer’s refractive index demonstrates the feasibility of using this configuration as a high resolution GMR-based sensor (up to 1 × 10−5 refractive index unit). The improvement is achieved through the waveguide properties with no perturbation of the grating parameters. In this scheme, coupling to a cladding mode increases the sensitivity ∼32 times that of coupling to a guided mode when using high-index substances. For low-index substances, exciting super mode resonances enhances the sensitivity by a factor of 6 compared to the guided-mode resonance.