Starting from the unified theory of low-frequency fluctuation, dissipation, and relexation processes, we studied the ultrasonic absorption in the thermal activation pro-cess in glasses. The theory involves only a single relaxation time and differs from the theory of distributed relaxation times. Nontheless, it can explain the experimental cha-racteristics of the ultrasonic absorption in glasses and the universality of these charac-teristics independent of kinds of materials, which couldn't be interpreted by the pre-vious theory with the distribution of relaxation times.