The influence of annealing above the glass transition temperature (T g) on chain entanglement and molecular dynamics of solution-cast poly(methyl methacrylate)/poly(styrene-co-maleic anhydride) (PMMA/SMA) blends was investigated via a combination of dynamic rheological measurement and broadband dielectric spectroscopy. Chain entanglement density increases when the annealing temperature and/or time increases, resulting from the increased efficiency of chain packing and entanglement recovery. The results of the annealing treatment without cooling revealed that the increase of the entanglement density occurred during the annealing process instead of the subsequent cooling procedure. Annealing above Tg exerts a profound effect on segmental motion, including the transition temperature and dynamics. Namely, Tg shifts to higher temperatures and the relaxation time (τmax) increases due to the increased entanglement density and decreased molecular mobility. Either Tg or τmax approaches an equilibrium value gradually, corresponding to the equilibrium entanglement density that might be obtained through the theoretical predictions. However, no obvious distribution broadening is observed due to the unchanged heterogeneous dynamics. Furthermore, side group rotational motion could be freely achieved without overcoming the chain entanglement resistance. Hence, neither the dynamics nor the distribution width of the subglass relaxation (β- and γ-relaxation) processes is affected by chain entanglement resulting from annealing, indicating that the local environment of the segments is unchanged.