The molecular relaxation dynamics of PMMA/PVDF blends above the glass transition temperature (T-g) over a wide composition range are studied by mechanical spectroscopy combined with differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) measurements. The mechanical spectra of the blends reveal the existence of two relaxation modes: alpha, ascribed to the glass transition, and alpha', related to the softening dispersion composed of the sub-Rouse modes and the Rouse modes. At phi PVDF = 40%, both the alpha and alpha' relaxation processes shift to low-temperatures and are accelerated, which is due to the formation of the interphase and unfavourable interchain entanglements in the intermediate composition. The abnormal dynamics of the blend at phi PVDF = 40% is further confirmed by the observed weak interaction between PMMA and PVDF from FTIR measurements and an obvious drop of the intermolecular coupling from the Coupling Model. However, the longer alpha'relaxation shows a different dynamic behavior from the alpha relaxation for the blends with increasing the PVDF content at phi PVDF <= 40%, which is due to the structure evolution and the change of chains entanglement with heating. This work enriches the understanding of the complex relaxation dynamics and the structure evolution in PMMA/PVDF blends. (C) 2019 Author(s).