Poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene) (PES) and poly(m-phenyleneisophthalamide) (aramid PMIA) were blended readily by the polymerization blending method. That is, N,N’-bis(trimethylsilyl)substituted m-phenylenediamine was reacted with isophthaloyl chloride in a PES solution in N,N-dimethylacetamide, and the resulting mixture was directly cast into films. The procedure afforded transparent films over the entire PES/PMIA composition range. Homogeneous mixing of the component polymers in the form of films was shown by a single glass transition temperature except for PES/PMIA 80/20, while a conventional solution blending method afforded less compatible blend films. This was also supported by the morphological studies. The tensile modulus and tensile strength of the blend films were improved over the entire PES/PMIA composition range by using the polymerization blending method. The polymerization-blended films had a high elongation at break over a relatively wide composition range from PES/PMIA 30/70 to PES/PMIA 60/40, leading to tough and ductile films. Annealing the blend films afforded high temperature resistant films with high tensile modulus and strength due to both crystallization and partial crosslinking of the aramid component.