A high-aspect-ratio platelet sodium niobate (NaNbO3) was synthesized by a topochemical reaction. Plate-like layered-perovskite Bi2.5Na3.5Nb5O18 (BNN5) as a precursor was first prepared, and then Bi3+ in the precursor phase was replaced by Na+ through topotactic conversion and perovskite NaNbO3 formed at 950 degrees C in NaCl molten salt. NaNbO3 crystals had an average length of 15 mu m and a thickness of 0.5 mu m. Scanning electron microscope and high-resolution transmission electron microscope analyses indicated that the retro-synthesis process of the desired perovskite NaNbO3 retained the morphological and structural features of the BNN5 precursor. It may be possible to synthesize other perovskite-structured templates using the same method.