It is pointed out first what favorable effect the small quantity of glass fiber ingredient in the reinforced Nylon 6 will have on the material to arrest the propagation of fatigue crack in it, and then the problem what is to account for the said favorable effect is carefully examined, from the standpoint of fractography, by performing repeatedly the test of low cycle fatigue, which is due to the tensile loads, by the use of an apparatus for the test of constant load. The results obtained are as follows.