When subjected to creep test, aluminum exhibits characteristics which are analogous to those of thermorheologically simple material in the sense that on a plot of logarithm of creep function versus logarithm of time the curves corresponding to different stresses have the same shape so that they all could be shifted to form a master curve. Based on this observation and in analogy with the idea of time‐temperature superposition, a theory for viscoelastic materials is developed in which the natural time appearing explicity in the argument of relaxation or creep function is replaced by a suitable scalar‐valued function of stress tensor.