A fibrillar-shaped metastable smectic phase forms during orientation-induced crystallization of poly(ethylene terephthalate). We investigated the effects of dimensions and regularity of the smectic phase on the mechanical properties by high-precision X-ray measurements. The drawing stress dependence was 76-168 MPa and the spinning speed dependence of 250-2000 m/min. The proportion, persistence length, and thickness of the smectic phase achieved their maximum values at 0.3-0.4 ms after necking. At this time g(II) also reached a minimum value of 3.7%-4.3%. The persistence length increased linearly with the natural draw ratio of as-spun fibers at less than 1500 m/min. The maximum values of the proportion, persistence length, and thickness showed no dependence on the drawing stress; however, the proportion and persistence length increased more rapidly. The increase of d-spacing with drawing stress led to an apparent elastic modulus of approximately 40 GPa for the oriented molecular bundle.