We present calculations on the spontaneous emission rates of Si rectangular quantum wires grown in the {100} plane. The electron and holes states are calculated within an effective mass approximation model. For holes, the minimum of the one-dimensional sub-bands is at the Γ- point. For electrons, the six anisotropic valleys at the conduction band minimum of bulk Si are not equivalent in the quantum wires and so, the minima of the energy sub-bands depend on the growth direction of the wire. For some wire directions in the {100} plane, the minimum of the ground electron sub-band is at the Γ-point. The directional dependence of the electron states is reflected on the spontaneous emission of the quantum wires. Light emission from Si quantum wires of diameters of a few nanometers can have direct transition character in the visible range. Phonon assisted transitions are then also present, but their intensity is three orders of magnitude smaller than the intensity of direct transitions. In Si quantum wires with indirect band gap the intensity and the peak position of the phonon assisted transitions exhibit dependence on the wires direction.