Based on binary optics, a new binary optic liquid crystal blazed grating was proposed by combining wave optics and liquid crystal orientation vector theories. Step-shaped phase was obtained by applying a step-shaped voltage on liquid crystal grating electrode and its diffraction efficiency is derived. By changing the number of steps in one unit, the diffraction angle is changed. The maximum diffraction efficiency can exceed 90% and form blazed grating at this order. The range of the first order of diffraction angle is 0°～10°.