Short glass fiber/poly(butylene terephthalate) composites with different glass fiber treatments were characterized within a temperature range from −150°C to + 150°C by dynamic tests in torsion and tensile modes. Untreated fibers or fibers coated by silane coupling agents associated or not with sticking agents were used. It was observed that the magnitude of β and α relaxations depends strongly on the microstructural parameters of such composites: fiber content, degree of crystallinity of the matrix, fiber length distribution, level of adhesion.