Observations of the expanding vapor cloud generated by the hypervelocity impact have been carried out. Under usual laboratory conditions, rock or ice materials cannot be vaporized completely. In order to investigate the expansion of a large amount of the vapor by complete vaporization caused by hypervelocity impact, we used a nylon projectile and measured the fundamental quantities of the vapor, that is, the expansion velocity, the shape of the vapor cloud, the temperature, and the light energy, by observing the light radiating from the vapor. The expansion velocity of the leading edge of the vapor is almost constant with time and is about twice that of the isothermal sound velocity, which is defined from the pressure and the density of the nylon projectile after passing of the shock wave. The shape of the cloud is not always hemispherical but depends on the combination of the material of the projectile and target, and impact velocity and angle. Inside the vapor cloud are not only the vapor but also the solid ejecta, which travel with the same velocity as the leading edge of the cloud. The temperature of the cloud in the case of nylon impacting on nylon decreases with the power law exponent of time of -0.04 to -0.2. The light energy radiating from the cloud is about 10-4 to 10-5 times the initial projectile kinetic energy. ¿ American Geophysical Union 1996