The contribution of water to DNA conformational change, DNA-protein interactions, and DNA-ligand recognition has been underlined. Here, we investigate the relaxation process of water around the Dickerson-Drew B-DNA duplex using terahertz spectroscopy. By controlling the DNA concentration, the relaxation time for water reorientation and the width of the DNA hydration layer are determined. We found that a layer of weakly bound hydration water, with a relaxation time of 11.6 ps, extends 7.6 angstrom from the tight-binding water layer. This result demonstrates that a shell of hydration water, with reorientation time on the order of tens of picoseconds, exists beyond the structurally integrated water usually observed around DNA. (C) 2015 Published by Elsevier B.V.