Si nanowires (SiNWs) have been produced by thermal vaporization on Si(111) substrate without catalysts added. The grown SiNWs have been characterized by Raman scattering, SEM, XRD, and electron diffraction and shown to be highly crystalline with only little impurities such as amorphous Si and silicon oxides. Photoluminescence (PL) study has illustrated that the Si band-to-band gap increases from 1.1 eV for bulk Si to 1.56 eV for the as-grown SiNWs due to quantum confinement effect. A strong PL peak at 521 nm (2.37 eV) is attributed to the relaxation of the photon-induced self-trapped state in the form of surface Si-Si dimers, which may also play an important role in optical limiting of SiNWs with 532-nm nanosecond laser pulses. With the observation of optical limiting at 1064 nm, nonlinear scattering is believed to make a dominant contribution to the nonlinear response of SiNWs.