The mutual diffusion process and interphase development taking place at an asymmetrical polymer-polymer interface between two compatible model polymers, poly(methyl methacrylate) (PMMA) with varying molecular weights and poly(vinylidene fluoride) (PVDF) in the molten state, were investigated by small-amplitude oscillatory shear measurements. The rheology method, Lodge-McLeish model, and test of the time-temperature superpositon (tTS) principle were employed to probe the thermorheological complexity of this polymer couple. The monomeric friction coefficient of each species in the blend has been examined to vary with composition and temperature and to be close in the present experimental conditions, and the failure of the tTS principle was demonstrated to be subtle. These were attributed to the presence of strong enthalpic interaction between PMMA and PVDF chains that couples the component dynamics. Hence, a quantitative rheological model modified from a primitive Qiu-Bousmina's model that connected the mobility in the mixed state to the properties of the matrix was proposed to determine the mutual diffusion coefficient (D-m). The modified model takes into account the rheological behavior of the interphase for the first time. In turn, viscoelastic properties and thicknesss of the interphase have been able to be quantified on the basis of the modified model. Effects of the annealing factors like welding time, angular frequency, temperature, and the structural properties as well molecular weight and Flory-Huggins parameter (chi) on the kinetics of diffusion and the interphase thickness and its viscoelastic properties were investigated. On one hand, D-m was observed to decrease with frequency until leveling off at the terimnal zone, to depend on temperature obeying the Arrhenius law, and to be nearly independent of PMMA molar mass, corroborating the prediction of the fast-mode theory. On the other hand, the generated interphase which reached dozens of micrometers was revealed to own a rheological property approaching its equivalent blend. Scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM-EDX) and transmission electron microscopy(TEM) were also carried out and confronted to the rheological results. Comparisons between mathematical modeling of concentration profile based on the D-m obtained from rheology and the experimental ones of SEM-EDX and TEM were conducted. Thus, a better correlation between theory and experimental results in terms of mutual diffusion and the interphase properties was nicely attained. The obtained data are in good agreement with literatures using other spectroscopical methods.