A photonic crystal (PhC) waveguide platform based on ring-shaped holes in a silicon-on-insulator substrate is proposed in order to realize a refractive index sensor with an improved sensitivity. The three-dimensional finite-difference time domain method is used to analyze the proposed design. The sensitivity is estimated by measuring the shift in the upper band-edge of the output transmission spectrum. Sensitivity analysis of a conventionally designed PhC waveguide, followed by modification of the structure, has been carried out for improving the sensitivity by introducing a row of holes that forms the line defect. Further improvement in sensitivity is obtained by replacing the defect row of holes by ring-shaped holes, which shows a significantly high sensitivity along with considerable output signal strength. The optimized design shows a wavelength shift of 210 nm for a change in ambient refractive index from air () to xylene (), corresponding to an average sensitivity of .