We report here the first study of formation kinetics of high-density vertical Si nanowire arrays formed on (111)Si substrates in aqueous AgNO3/HF etching solutions. From TEM and electron diffraction analyses, the as-prepared Si nanowires were determined to be perfectly single crystals and the axial orientation of the Si nanowires was identified to be parallel to the  direction, which was identical to the orientation of the (111)Si wafers used. The typical widths of the Si nanowires prepared are in the range of 25-150 nm. The lengths of the Si nanowires were found to increase linearly with the reaction time in samples treated at 5-30 degrees C. By measuring the growth rate at different temperatures, the activation energy for the linear formation of Si nanowire arrays on blank-(111)Si was obtained from an Arrhenius plot to be about 0.38 eV. The results of the contact angle measurements further indicated that the hydrophobicity of the Si samples was enhanced significantly by the formation of Si nanowire arrays. The hydrophobic behavior of the HF-treated Si nanowire arrays could he explained by Cassie's model.