We have successfully measured electron transport nanostructures of conjugated polymer thin films by conductive atomic force microscopy, using an air-stable electron-injecting electrode coated with ethoxylated polyethylenimine. Electron- and hole-transport networks in donor/acceptor polymer blends can be selectively observed by using an appropriately coated electrode. This approach enables us to visualize phase-separated nanostructures of donor/acceptor polymer blends for thin-film electronic devices based on their semiconducting properties.