OptiFDTD and Fast Fourier Transform method are used to simulate and analyze the diffracted field of transmission sub-wavelength binary gratings (TSBG). And the influence of grating parameters on the diffraction efficiency is numerically analyzed. The analysis result demonstrates that, for TSBG, the normal incident TE mode beam can be diffracted to the +1st order, the interval number has few influence on diffracted angle, the performance of a TSBG is determined by the grating period, material index and interval number for a given wavelength.