Ultrafine one-dimensional (1-D) semiconducting nanostructures with diameters below 10 nm are attracting great research attention. Using a laser-ablation chemical vapour deposition (CVD) method, we reported the synthesis of single-crystal In2O3 nanowires with diameter below 4 nm. The as-synthesized ultrathin In2O3 nanowires act as the ultrathin branches of hierarchical In2O3 nanostructures and show fast photoinduced switching surface wettability behaviors and the contact angle decreased from 134.3^o to 0^o in 10 min. Transparent thin-film transistors (TTFTs) were fabricated using the as-synthesized product and the device conductance was 1-2 orders higher than the average conductance of the In2O3 single nanowire devices, revealing good opportunity in transparent electronics.