An automatic system for photoelastic stress analysis is proposed. This system is composed of a photodiode, an amplifier, an A-D converter and a computer. Analogue information on isochromatic fringe patterns is converted to digital one by the photodiode the amplifier and the A-D converter. Thus, by using the microcomputer, we obtain stress distributions from these digital data of fringe patterns. We applied this system to determine stress distribution in circular and square plates subjected to concentrated loads, and obtained good results.