We have investigated optical characteristics of silicon nanowire (Si NW) on Al disk arrays using the finite-difference time-domain (FDTD) simulations. Without the Al disk, the Si NW arrays alone exhibit strong absorption peaks, originated from guided mode resonance. The arrays of SiNW with Al disk possess additional broad peaks, at slightly larger wavelengths than those of the resonant guided mode peaks. The FDTD simulations show formation of concentrated electromagnetic field at the Si NW/Al interface, indicating excitation of localized surface plasmons. These results suggest that bottom-contact electrodes can work as means to enhance the optical absorption of the Si NWs as well as to collect carriers in Si NW-based optoelectronic devices.