The sound absorption coefficient (α) of trees was theoretically derived and expressed as α=G·f^1/2, where f is frequency and G is a constant; since the value of G cannot be calculated directly, absorption coefficients of four kinds of trees were experimentally measured by the reverberation-chamber method. It was found that sound energy was absorbed mainly by the leaves of trees and not their trunks. The values of G were determined by comparing theoretical and experimental results, and were found to be between 0.001 and 0.002. The attenuation in vegetation was theoretically expressed by G. To check the accuracy of this, experiments were carried out using two kinds of trees. The sound energy absorbed by tree leaves was measured. The theoretical values agreed approximately with experimental results, indicating that the attenuation through absorption can be predicted.