A method for controlling a dispersion size of an elastomer in a thermoplastic elastomer composition having improved durability and impermeability obtained by melt-mixing (A) a halogenated isobutylene elastomer, (B) polyamide and (C) a dispersion aid, and dynamically vulcanizing the resultant blend to form the dynamically vulcanized polymer blend, wherein the halogenated isobutylene elastomer is dispersed in the polyamide matrix at a volume-average dispersion diameter of less than 2.0 .mu.m.