Abstract: In this paper, a high-diffraction efficiency Dammann grating with sub-wavelength structure was designed and prepared based on the Rigorous Coupled-Wave Analysis (RCWA) theory and optimized by the genetic algorithm. The array number of diffractive spots was 11 and the minimum feature size achieved 0.95 m. The design value of diffraction efficiency can achieves 95%, around 15% more than conventional Dammann grating while the uniformity can be less than 2%. By using Electron Beam Direct Writing and Reactive Ion Etching, the sub-wavelength structure was patterned on the silica basement. The experimental results show that the sub-wavelength structure with nanometer resolution can be patterned by electron beam scanning exposure. In reactive ion etching, the etching rate and the grating line-shape can be affected by radio frequency (RF) power, system pressure and gas flow, and the line-width error in Electron Beam Direct Writing was also analyzed.