The viscoelastic properties of carbon nanofiber (CNF) reinforced polymer matrix composites are examined in this chapter. Temperature-dependent variations in the properties from subzero to above glass transition temperature (Tg) is studied. Different compositions of CNF/polymer composites are evaluated for parameters such as storage modulus, loss modulus, and damping parameter using a dynamic mechanical analyzer. The maximum use temperature and Tg of such composites are also determined. The ability to tailor the properties by means of CNFs and improve the stability of reinforced polymer composites at high temperatures is important for aerospace applications that handle mechanical load under thermal extremes.