Time-resolved small-angle X-ray scattering measurements using synchrotron radiation have been conducted during tensile deformation of a poly(ethylene terephthalate) fiber at room temperature. The marked change of the scattering patterns from the layer-line scattering with a four-spot nature to the layer-line scattering concentrated on the meridian at small fiber strains could be interpreted as the decrease in the disorder of the long-period structures without assuming the tilted lamella stack. The microvoids formed at a large fiber strain were characterized by applying a method proposed by the present author to the equatorial streak and the relative microvoid volume fraction, size and shape of the microvoid cross sections and longitudinal size of the microvoids were determined. It was inferred that the microvoids were formed in the regions which linked long-period structures, depending on the properties of these regions and the formation of microvoids contributed to the fracture toughness of the fiber..