Abstract: The design of fused silica transmission blazed grating based on modal method is presented under Littrow mounting. A -1st transmitted order blazed grating is designed at the wavelength of 1.06 μm. The first two modes (i.e. mode 0 and mode 1) propagate through the grating with different effective indices and couple out at the grating-substrate interface. And a high diffraction efficiency of -1st transmitted order can be achieved if the first two modes have a phase difference of odd multiple of π. The diffraction performance of the blazed grating is analyzed by the rigorous coupled-wave analysis theory. The design of blazed grating based on modal method matches the analysis perfectly.