In this study, we investigated the optical properties of sub-wavelength a-Si thin film transmission gratings, especially the polarization effect, the phase difference and the birefringence by using linearly polarized He-Ne laser beam (632.8nm). The a-Si transmission grating of the thickness $of < 0.1 mum$ with four-type period($Lambda = 0.4 mum and 0.6 mum$ for sub-wavelength and $Lambda = 1.0 mum and 1.4 mum$ for above-wavelength) on quartz substrates have been fabricated using 50 KeV Ga+ Focused-Ion-Beam(FIB) Milling and $CF_4$Reactive-Ion-Etching(RIE) method. Finally, we obtained the trating array of a-Si thin film with a period $0.4 mum, 0.6 mum, 1.0 mum, 1.4 mum$ which have nearly equal finger spacing and width, sucessfully. Especially, for gratings with $Lambda = 0.6 mum(linewidth=0.25 mum, linespace=0.35mum), the etamax at heta_в=17.0^{circ}$ is estimated to be 96%. As the results, we believe that the sub-wavelength grating arrayed a-Si thin film has the applicability as the optical device and components.