Crystal structure and phonon dynamics up to 150 meV of the spin-Peierls material CuGeO3 have been investigated by neutron scattering experiments. We found a large lattice anomaly in the a-b plane, vertical to the one-dimensional antiferromagnetic spin chain, associated with additional diffraction peaks in the b∗-axis at temperatures much higher than the spin-Peierls transition temperature. The phonon modes of the chain oxygens at 100 meV show consistent evolution with diffraction. Therefore, we conclude that the structural evolution of the chain oxygens in the a-b plane, which mediates the spin-spin interaction along the c-axis, drives the spin-Peierls transition. © 1994, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.