The dynamic mechanical behaviour of a series of poly(butylene terephthalate)/glass beads composites was studied as a function of filler ratio, filler size and surface treatment. Samples were subjected to compression-tension testing within a temperature range from -150°C to 150°C. The real part (E') of the dynamic modulus and the loss tangent (tan δ) were studied around β and α relaxations. It was observed that the magnitude of these relaxations decreased as the glass beads ratio increased. A similar decrease was observed at constant filler ratio with improving filler-matrix adhesion by a coupling agent. All these results might be due both to the matrix substitution by fillers and to the reducing of polymer chains molecular mobility in the vicinity of the interface. Using Hashin's correspondence principle, the viscoelastic behaviour of such composites was described by self-consistent models in the neighbourhood of the α relaxation and compared to experimental data.