We report the effect of H-2 ambient annealing on the microstructure and vibrational properties of silicon nanowires (SiNWs) grown by atmospheric pressure chemical vapor depositions. The SiNWs were characterized by Fourier Transform Infrared Spectroscopy (FTIR), Field-Emission Scanning Electron Microscopy (FESEM) and High- Resolution Transmission Electron Microscopy (HRTEM). The HRTEM study revealed that the thickness of oxide sheath surrounded by core silicon decreased with increasing H-2 ambient annealing and consequently the vibrational spectra were changed. In FTIR spectra, the transverse optic and longitudinal optic peak positions of Si-O symmetry and asymmetry showed a blue shift of the outer oxide of SiNWs. The Si-O-Si peak position remained unchanged at 1080 cm (1) while the integrated absorption of Si-O-Si vibration band decreased with increasing H-2 flow rate. (C) 2010 Elsevier B. V. All rights reserved.