In many applications, where the period of a grating structure is less than the wavelength of the incident light, the grating structure will function more like a medium of uniform index of refraction than as a normal grating. The effective medium index being equivalent to the volume mean index of the grating structure. The equalivent refractive index will depend on its structure or material. When the structure is given orientation, anisotropy called constitutive birefringence will be generated. A refractive index profile can be also be provided on material surface. Making use of these properties of sub-wavelength structure, we have successfully established methods to design, fabricate, prototype, and evaluate elements which perform polarization separation. The polarization separation elements consist of two-layers, plastic material (low refractive-index layer) and vacuum evaporation material (high refractive-index layer). The grating structure has a very small depth. Polarization separation elements both for single wavelength and two wavelengths can be designed. These elements can be designed for any fixed incident angle to the substrate, They can replace glass polarization separation elements and half mirror elements currently used for DVD/CD. It is also one of advantages that they can be mass produced with low cost.