In order to investigate changes in fine structure of nylon tire cord subjected to repeated extensions, two kinds of nylon tire cords having different thermal shrinkage were used. Repeated extension fatigue test of nylon tire cord was carried out under a cyclic extension of 2% with a frenquency of 3 Hz at room temperature. To investigate changes in physical properties, and fine structure, measurements in molecular weight, density, birefringence, WAXD, tensile property and the dynamic viscoelasticity were carried out. At the initial fatigue step (10,000 fatigue cycles), amorphous orientation is increased with fatigue cycles because of strain-hardening effects. But at higher fatigue cycles, amorphous orientation is decreased because of chain scisson effects. With the increasing fatigue cycles, molecular weights are decreased remarkably, but change of density is little, so we supposed that the strain-hardening and chain scisson occur at the same time in the amorphous region. Fine structure of N2 sample having lower shrinkage is less changed than that of Nl sample having higher shrinkage. So, it is supposed that the amorphous region of N2 sample is more stable than that of Nl sample with repeated extensions.