Silicon nanowires (SiNWs) have been produced by a simple thermal heating method with gold as a catalyst. The grown silicon nanowires were highly crystalline with little impurities such as amorphous Si and silicon oxides. Photoluminescence (PL) study has indicated that the Si band gap increases from 1.1 eV of bulk Si to 1.59 eV for the as-grown SiNWs due to quantum confinement effect. A strong PL peak around 540 nm (2.28 eV) is attributed to the relaxation of photon-induced self-trapped state in the form of surface Si-Si dimmers, while the blue light emission around 390 nm is attributed to the silicon oxide impurity on the SiNWs surface.