We report on the effect of the addition of phosphotungstic acid (PWA) in Nafion membrane on ethanol-crossover and the proton conductivity for DEFC application. A set of PWA-Nafion composite membranes (PWA 0, 5, 10, 15, 20 wt%) was prepared by solution casting and their microstructures, diffraction patterns, permeability, and proton conductivity were systematically characterized. The significant reduction in ethanol-crossover is observed with increasing PWA concentration in PWA-Nafion membranes, which is mainly attributed to an improvement in crystallinity of the membrane. PWA provides additional nucleation sites during solidification leading to higher crystallinity, which is supported by the membrane permeability tests. The proton conductivity of the composites is enhanced with PWA concentration until 15 wt% due to an increase in hopping pathways, while higher PWA of 20 wt% leads to a conductivity decrease possibly due to the excessive particle aggregations that limit ion transports. These PWA-Nafion composites were implemented in prototype DEFC devices as a membrane and the maximum power density achieved was 22% higher than that of commercial Nafion-117 device.