Abstract: A novel nano-grating coupler based on silicon-on-insulator structure is proposed to solve the mismatch issue between the silicon-based optical waveguide and the single-mode fiber. According to the principle of collimation, the structure model and characteristics of the nano-grating coupler are investigated by using the finite-difference time-domain method. Optiwave OptiFDTD 8.0 software is used to investigate the factors related to the coupling efficiency, such as the incident wavelength, the incident angle, the grating cycle, the grating duty ratio, the etching depth, the buried-oxide layer thickness and so on. Optimum parameters of the grating structure are obtained. The structure is optimized by adding the rear reflector, the antireflective film and the bottom dielectric reflector on the base of the basic grating. The maximum coupling efficiency of the grating coupler is up to 59.37% with a 3 dB bandwidth of 65 nm.