Electronic band structure and absorption coefficient for intersubband transitions in a Si/Si1−xGexsuperlattice under external electric fields are reported using a full Brillouin‐zone energy‐band description. Full spectra of the absorption coefficient are calculated from zero to high electric fields. The effect of Wannier–Stark localization on intersubband transitions is theoretically studied for the superlattice and the charge densities are given for various electric fields. A finite‐length superlattice instead of an infinite superlattice is considered to show the effects of electric fields on the optical absorption. The Stark shift is observed toward the shorter photon wavelength for an intersubband transition in a quantum well.