The present invention discloses a nanowire fabrication method and a semiconductor element using a nanowire fabricated thereby. The method of the present invention comprises steps: providing a substrate; sequentially depositing a silicon dioxide layer and a silicon nitride layer on the substrate; forming a patterned photoresist layer on the silicon nitride layer; using the patterned photoresist layer as a mask to etch the silicon nitride layer and the silicon dioxide layer with the substrate partly etched away to form a protrusion; removing the patterned photoresist layer to form an isolation layer; removing the silicon nitride and the silicon dioxide layer, sequentially depositing a dielectric layer and a polysilicon layer; and anisotropically etching the polysilicon layer to form nanowires on a region of the dielectric layer, which is around sidewalls of the protrusion.