Abstract: The molecular relaxation dynamics in poly(vinyl acetate) (PVAc) was studied by mechanical spectroscopy above the glass transition temperature (Tg). The temperaturedependent mechanical spectra reveal the existence of two relaxation modes: α, ascribed to the glass transition, and α′, which might be related to the liquidliquid transition. The relaxation time follows VogelFulcherTamman law for both of the relaxation modes. A detailed analysis of the frequencydomain spectra above Tg shows a dynamic crossover at temperature TB ≈387 K through the temperature dependence of relaxation time, relaxation strength, and shape parameter. According to the phenomenological coupling model, the crossover is suggested to be caused by the strong increase of intermolecular cooperativity below TB with decreasing temperature.