A highly sensitive sensor design based on two-dimensional photonic crystal cavity is demonstrated. The geometric structure of the cavity is modified to gain a high quality factor, which enables a sensitive refractive index sensing. A group of slots with optimized parameters is created in the cavity. The existence of the slots enhances the light–matter interactions between confined photons and analytes. The interactions result in large wavelength shifts in the transmission spectra and are denoted by high sensitivities. Experiments show that a change in refractive index of ?n???0.12 between water and oil sample 1 causes a spectral shift of 23.5?nm, and the spectral shift between two oil samples is 5.1?nm for ?n???0.039. These results are in good agreement with simulations, which are 21.3 and 7.39?nm for the same index changes.