Polypropylene (PP) and five kinds of monodisperse polystyrene (PS) with different terminal relaxation times were used to explore the relationship between the mobility of polymer molecular chains and the coarsening process of immiscible polymer blends with a co-continuous morphology under quiescent melt annealing at different temperatures. The terminal relaxation time of all neat PP and PS was determined by a rheological approach to characterize the mobility of molecular chains. A selective dissolution experiment showed that all PP/PS (50/50) blends maintained a co-continuous structure during the whole annealing process. Significant coarsening behaviors were observed for all PP/PS blends under a scanning electron microscope. A linear time dependence of the size of the PS phase was found in all PP/PS blends and the coarsening phenomenon was more obvious with the decrease of the terminal relaxation time of the PS phase because of the increase of the mobility of the polymer molecular chains. A direct relationship between the phase coarsening rate and the terminal relaxation time of the PS phase was found for the first time and it satisfied the equation . According to this equation, the formulae and k ∝ Mw-1 can be derived, which can provide significant information for the control of the phase coarsening process of immiscible polymer blends with a co-continuous morphology.