A new Differential Interference Contrast (DIC) method is proposed based on the DIC theory by replacing the two Nomarski prisms of conventional DIC with bi-frequency blazed gratings, which overcomes the drawbacks of artifacts in the conventional DIC technology. The Bi-frequency blazed gating has been designed, and intensity distributions of plane wave through the grating also have been numerically simulated. The pure phase object and its image are simulated by utilizing grating DIC, and the image agrees with the phase profiles of object very well. Analysis of the influence of shear and bias on the contrast of image demonstrates that image contrast is obviously enhanced when a bias retardation is introduced, and the smaller shear, the higher contrast.