Metallic nanoparticle-decorated silicon nanowires showed considerable promise in a wide range of applications such as photocatalytic conversion, surface-enhanced Raman scattering, and surface plasmonics. However there is still insufficient amount of Raman scattering in Si nanowires with such decoration. Here we report the heteroepitaxial growth of Ag nanoparticles on Si nanowires by a surface reduction mechanism. The as-grown Ag nanoparticles exhibited highly single crystallinity with a most probable diameter of 25 nm. Raman scattering spectroscopy showed a new sideband feature at 495 cm(-1) below the first order Si transverse optical Raman peak due to HF etching. This new feature sustained after sequential surface treatments and rapid thermal annealing, therefore was attributed to polycrystalline defect at subsurface, which was confirmed by high-resolution transmission electron microscopy observations. Correlated atomic force microscopy and Raman mapping demonstrated that single Ag nanoparticle decoration significantly enhanced Raman signal of Si nanowire by a factor of 7, suggesting that it would be a promising approach to probe phonon confinement and radial breathing mode in individual nanowires down to sub-10 nm regime.