Results of a study on the modifying mechanical properties of loose optical-fiber poly(butylene terephthalate) (PBT) tubes, produced during the standard industrial extrusion process, show that heat treatment make the structure of their material to change. The study comprised measurements of mechanical strengths properties of the tubes (tensile strength, compression strength, kinking) and determination of tube material structure [by differential thermal analysis (DTA), wide angle X-ray scattering analysis (WAXD), and scanning electron microscopy (SEM)]. Results of the study allowed observation that the annealing at 70degreesC for 34 h of the tubes caused the crystalline a phase to increase in the tube material from similar to28.5% to similar to31.5% and the structure of the existing crystallites to become more perfect. This made the values of certain mechanical properties of the tubes to increase even by as much as 30%. The tubes following such thermal treatment could be used in cables exposed to heavy-duty operation in arduous environments, where a larger margin from the standpoint of mechanical properties is required.