Sub-wavelength gratings can be applied in the design of various optical devices for the special reflection characteristics. The band structure,eigenmodes and reflection spectra of sub-wavelength dielectric gratings are simulated with finite-difference time-domain (FDTD) method and scattering-matrix method. Simulation results show that the resonant peaks of the reflection spectra originate from the leaky modes and eigen modes excited by external incident waves are different between normal and oblique incident conditions. Further more,the reflection spectra line shape of the grating can be controlled by controlling the frequencies and Q factors of the leaky modes. In the experiments,sub-wavelength gratings in silicon-on-insulator (SOI) substrate are fabricated by electron beam lithography and reactive ion etching methods. Experimental reflection spectra of the fabricated grating agrees well with the simulation results.