Improvements of ionic conductivity of perfluoroalkyl sulfonated polymer membranes by a film recasting method with several kinds of additives are reported. Nafion film was liquefied by dissolving in ethanol/water (50/50) at 230 degrees C in a high pressure vessel to make a 10% polymer solution. This solution was cast on a glass petri dish with a solvent dimethylformamide, together with some additives, and the film was dried in an oven at 70 degrees C for 2 h, then at 130-150 degrees C in a vacuum. The resulting film was cut and mounted on an electrochemical cell for AC impedance measurements. The conductivity of the films was compared with that of original Nafion in dry and wet states. The effective increase in ionic conductivity was observed for three kinds of additives: the moisture absorbing reagents such as molecular sieve, organic sulfonate compounds such as alkylbenzene sulfonates, and structure aligning copolymers with ion exchange sites. These results offered supporting data to the criteria for designing high performance ion conducting membranes, which were proposed in a previous study based on the ion and water transport model in Nafion membranes [1].