Equilibrium molecular dynamics simulations were carried out in order to determine the viscosity of water confined into carbon nanotubes. We have found that the viscosity of confined water is about an order of magnitude lower than bulk and increase non-linearly with nanotube diameter. We quantify the influence of density of water upon its viscosity, and observed a strong dependence between both quantities. After analysis of density profiles and diffusion coefficients we conclude that water at high density regime experiences a structural transition resulting in a large increment in viscosity. (C) 2015 Elsevier B.V. All rights reserved.