In situ spectro-ellipsometric measurement was used to obtain the various structural parameters that characterize Si/Si1-xGex multiple quantum wells grown on Si(100) substrates. The primary feature of the evolution of the ellipsometric angles (psi, Delta) in the time domain is a short-period oscillation of their signal levels resulting from the change in optical contrast when the top layer is alternately exchanged between Si1-xGex and Si. The top layer material and the number of layer pairs at each moment of growth can be derived from this oscillation. At transparent energies, the long-period modulation structure resulting from the optical interference between the whole multiple quantum well (MQW) and the Si substrate superimposes itself as an envelope curve on the,short-period oscillation. The overall growth rate of the MQW is derived from the modulation period. One can quickly discriminate two-dimensional (2D) planar growth and 3D island growth by examining the psi-Delta trajectories; 2D growth shows finite-size, closed-loop trajectories at 3.4 eV, whereas 3D growth shows drift motion of the oscillatory loops towards the region of lower psi and Delta angles.