The noncovalent interactions between encapsulated water chains and single-walled carbon nanotube (SWCNT) are studied using a self-consistent charge density functional tight binding method with dispersion correction. The most interesting and important feature we observe is the diameter shrinking of CNTs when water chains are confined inside SWCNT. The diameter shrinking of CNTs can be suggested to the original of the van der Waals and H-π interaction between water chains and CNTs. The calculated Raman spectra show the interactions between SWCNTs and water chains probably give rise to a kind of “mode hardening effect,” which agrees with the diameter shrinking of CNTs when water chains are confined inside SWCNT. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011.