An ultrahigh density array of epitaxial PbTiO3 (PTO) nanoislands with uniform size was fabricated on a single-crystalline Nb-doped SrTiO3 (100) substrate over a large area (cm^2 scale) by simple but robust method utilizing polystyrene-block-poly(4-vinylpridine) copolymer micelles. Each nanoisland has an average volume of 2.6 × 10^3 nm^3 (a height of 7 nm and a diameter of 22 nm). Because of uniform nanoislands over a large area, a synchrotron X-ray diffraction experiment was successfully employed to analyze the domain structures of PTO nanoislands. They showed well-defined epitaxy on the substrate, which was also confirmed by high-resolution transmission electron microscopy. All of the nanoislands existing in the entire area showed distinct piezoresponse that confirms the existence of ferroelectricity at this size. The results indicate that the critical size of ferroelectrics could be scaled-down further, thereby much increasing the density of ferroelectric devices.