In order to clarify the elementary processes of impact disruption, we conducted impact experiments with water ice at an impact velocity of 3.6 km/s and observed shock wave and fracture propagation in it by means of ultra-high speed photography. We observed that a region in which HEL (Hugoniot elastic limit) followed the elastic precursor wave, expanded with a velocity of 3--2.5 km/s until the pressure fell below 240 MPa. Below that pressure, a damage region appeared 0.8--3 &mgr;s after the passage of precursor wave. In this region, dynamic shear strength of water ice was estimated to be 21 MPa. Below 80 MPa, the several radial cracks proceeded toward the rear surface and broke the sample before the tensile fracture caused by reflection waves from an antipodal point became visible. Therefore, the main mechanism to cause the largest fragment is the radial crack growth rather than a spallation at the rear. ¿ 2000 American Geophysical Union