We present a technique for refractive index sensing using a phase grating structure. A grating under normal incidence can be designed such that the first-order diffracted light travels at a diffraction angle of 90° with respect to the zeroth order. The diffracted light, which is along the direction of periodicity, can further be diffracted from the grating and interfere with the zeroth-order light. Under this condition, the π phase difference that arises between the two interfering beams results in a transmission dip. We can tune this dip wavelength for senor applications, based on the grating equation. This Letter presents both simulation and experimental data that show good agreement with each other.