Abstract: Using the digital processing technique the photoacoustic (PA) signal of materials can be detected directly with a microcomputer rather than a Lock-in Amplifer. Some experimental results obtained by using cross-spectrum technique and many times average of the sampled signals are given in this paper. For the samples of carbon and yellow diamond powder, both the amplitudes and phases of the PA signals measured by the microcomputer Apple-II are in good agreement with those obtained by the Lock-in Amplifer PAR 5204. Besides, the thermal diffusivities of some opaque thin samples are measured with the above-mentioned method and obtained results are also shown in this paper. Comparing experimental results with those detected by a Laser Thermal Conductivity Detection System for the same samples, we will see a good agreement between them.