The differences in molecular fluctuation of protonated water composed of hydrogen isotopes are elucidated by ab initio path integral molecular dynamics simulation, in which 'on-the-fly' calculation of potential energy surface is done accurately with the MP2 electron correlation to sample quantum nuclear configurations. It turned out that, at the room temperature 298 K, H3O+ is quite flexible in which pyramidal structure is inverted frequently with multi-dimensional molecular distortion, while the flexibility of D3O+ is much more restrained. Our analysis shows that anharmonicity in full-dimensional rovibrational motion is essential in this isotope effect. (C) 2003 Elsevier Science B.V. All rights reserved.