In spite of the importance of vibrational Stark effect (VSE) and many attempts, origin of VSE is still unclear in molecular level. Here, we studied on origin of VSE of hydroxyl stretching vibration in small water clusters (monomer, dimer, and tetramer) assuming that VSE can be separated by nuclear and electronic contribution. We calculated total Stark tuning rate (Δμtot) and its nuclear contribution (Δμgeom) using the ab initio method, then the electronic contribution (Δμelec) was simply obtained by the difference, Δμtot – Δμgeom. In all cases, the nuclear contribution has dominant contribution to VSE. The hydroxyl stretching mode with neighboring hydrogen acceptor showed larger Δμgeom than that of dangling bonds. Furthermore, the calculated Δμgeom became larger in larger cluster due to the hydrogen bond network. The comparison between Stark tuning rates including and excluding anharmonicity supports the importance of potential anharmonicity in VSE, as previously reported. Interestingly, a good linear rel...