A decrease in the thermal conductivity and the optical absorption length of a silicon wafer damaged by ion implantation causes an increase in the amplitude of photoacoustic displacement (PAD) on the sample surface. The behaviors of the PAD were investigated by thermoelastic analysis. The theoretical results indicate that the amplitude of PAD is approximately proportional to the square of the thickness of the damaged layer and is characterized by the ratio of the optical‐absorption coefficient to the thermal conductivity of the damaged layer. Experimental results, which were quantitatively measured by a highly sensitive PAD interferometer, can be explained well by the theoretical predictions. The thermal conductivity of the damaged layer by implantation above critical amorphization dose can be estimated to be 1–3 W/mK by comparison of the experimental data with the theoretical results. ©1995 American Institute of Physics.