Various studies have so far been made of fatigue crack propagation about all kinds of engineering materials, but as its essential mechanism has not yet been known an experiment was performed on plane bending fatigue test using a Polycarbonate plate for specimen, and the same is reported hereunder in this paper.The relation that the differentials of the crack length due to the number of repetitions is proportional to the crack length holds with polycarbonate as it does with metals. The number of striations is equal to the number of cycles under high stress, but it is not equal to the number of cycles under low stress. The main crack is propagated by the repeated stress under high stress, but the crack is propagated after the propagating of shear yielding band for several number of cycles under low stress. The relation between the macroscopic propagating rate and the stress intensity factor range follows the formula presented by Paris for metals, and it is certified that the fatigue fracture mechanisms differ essentially under the high and low stresses.