A plan wave incident upon the grating will excite discrete modes. The modes with n2eff>0 can propagate through the gratings, while those with n2eff<0 are evanescent modes. The basic idea behind the simplified modal investigation is the excitation and coupling of the propagating modes in gratings. If an odd number multiple phase difference is accumulated for the first two propagating modes, the incident light will be diffracted into the －1st order with nearly 100 % diffraction efficiency. A highly efficient full polarization blazed grating with rectangular groove was designed theoretically by modal method in the resonance domain. The design procedure of the full polarization rectangular blazed grating could be divided into three steps. Step 1: present the grating period in the resonance domain. Step 2: present fill factor by the eigenvalue functions of the grating. Step 3: present groove depth based on the phase difference of the propagating modes. As an example, the design of the fused silica full polarization rectangular blazed grating was demonstrated for the wavelength of 633 nm, which has about 96.7% and 98.1% negative first-order diffraction efficiency for TE and TM polarizations, respectively. The full polarization blazed grating with fused silica has a wide-band incident angles and incident wavelengths, which especially has high fabrication tolerances.