In this paper, we report a method of fabricating silicon nano-wire based on thermal oxidation technique. In this method, we first fabricate a wider structure with traditional lithography, and then use a layer of silicon nitride as mask to oxidize silicon. At the same time, due to the lateral diffusion and oxidation of oxidant, silicon is consumed by oxidant and the width of the silicon structure will be reduced to nano range when we remove the silicon dioxide. The factors affecting the ratio of vertical and lateral oxidation, for example, the gentle slope caused by isotropic oxidation and the inhomogeneity of the sidewall of silicon nano-wire, are discussed at last. Our results should be useful in generating silicon–based nanospintronics devices with careful selection of the oxide parameters.