We report a simple technique to fabricate horizontal, uniform Si nanowire arrays with controlled orientation and density at spatially well defined locations on substrate based on in situ hard mask pattern formation approach by microphase separated polystyrene-b-poly(ethylene oxide) (PS-b-PEO) block copolymer (BCP) thin films. The methodology may be applicable to large scale production. Ordered microphase separated patterns of the BCP were defi ned by solvent annealing and the orientation was controlled by fi lm thickness and annealing time. Films of PEO cylinders with parallel orientation (to the surface plane) were applied to create ’frames’ for the generation of inorganic oxide nanowire arrays. These PEO cylinders were subject to selective metal ion inclusion and subsequent processing was used to create iron oxide nanowire arrays. The oxide nanowires were isolated, of uniform diameter and their structure a mimic of the original BCP nanopatterns. The phase purity, crystallinity and thermal stability of the nanowires coupled to the ease of large scale production may make them useful in technological applications. Here, we demonstrate that the oxide nanowire arrays could be used as a resist mask to fabricate densely packed, identical ordered, good fi delity silicon nanowire arrays on the substrate. The techniques may have signifi cant application in the manufacture of transistor circuitry.