Polycrystalline silicon (poly‐Si) ‘‘slit nanowire’’ was fabricated in a slit formed with 100 nm lithography, microwave dry etching of silicon substrate, conformable filling of the trench by chemical vapor deposition (CVD) SiO2, slit etching of the CVD SiO2, conformable deposition of doped amorphous silicon, followed by etchback and annealing. Observation with transmission electron microscope confirmed that a poly‐Si slit nanowire, with a cross section of ∼5–8 nm×20 nm is fabricated. Appropriate annealing of thea‐Si layer makes the poly‐Si grains grow to more than 2 μm in length. This technique would make it possible to realize silicon quantum devices, and to fabricate conventional integrated circuit devices and light emitting slit nanowire devices on a same silicon chip, which would allow the fabrication of integrated optoelectronic circuits.