Nano structuring of silicon surface by low energy ion irradiation is reported. Semi insulating n-Si (100) has been irradiated by 50 keV Ar+ ion beam at an angle of 50° with respect to surface normal. The irradiated sample's surfaces were analyzed by Atomic Force Microscopy. Irradiation caused formation of nano-sized elliptical dots aligned in rows perpendicular to ion beam direction at fluence of 1×1017 ions/cm2. At higher fluences of 3×1017 ions/cm2 and 7×1017 ions/cm2 self organized ripples were developed on the surface.