The structural features of a polymer electrolyte membrane (PEM), consisting of polystyrene sulfonic acid (PSSA) grafted onto poly(ethylene-co-tetrafluoroethylene) (ETFE), can be characterized semiquantitatively by atomic force microscopy (AFM). The cross-sectional AFM phase images are converted to the binarized image by fitting two Gaussian functions. The domains correspond to hydrophilic PSSA domains and hydrophobic ETFE crystalline and amorphous regions, respectively, at lower and higher phase shift values. The area fraction of PSSA domains was consistent with the volume fraction determined by the grafting degree (GD). The dependence of the radius and interdomain distance of the PSSA domains on the GDs of PEMs shows discontinuous features at the threshold GD (39%). The former slightly increased from 10 to 12 nm and significantly increased to 17 nm at a GD greater than 39%; the latter decreased from 140 to 54 nm with increases in GDs up to 39% but inversely increased to 78 nm at a GD of 46%. This discontinuous change in radius and interdomain distance should be caused by the fusion of adjacent PSSA domains to form a larger size and spacing and thus less connectivity between each large domain, thereby lowering the conductivity at GD greater than 39%. We were able to demonstrate the existence of an ion-conducting hydrophilic path with a radius of approximately 10 nm. Even though it has received little attention in the past, it is expected to enable the design of electrolyte membrane functions in the future.