The structural, electric and optical properties of hydrogen-passivated silicon nanowires with [001], [011], and [111] growth directions and diameter up to 3nm are studied from first principles. The effects of nanowire diameter, growth directions, surface structures, and relative stability of these nanowire systems are systematically investigated. We find that quantum confinement still plays an important role for diameter upto 3nm for all growth directions and surface structures. All factors such as nanowire diameter, growth direction, and surface structure are found to cause significant changes in energy band gap, DOS, and band structures of the nanowires.