We have investigated dielectric relaxation in aqueous ionic solutions by terahertz time-domain spectroscopy to elucidate the ionic hydration effect on the water structure. In the low frequency region (<1.0 THz), the dielectric spectra decreased in amplitude with decreasing size of dissolved cations, which indicates the deceleration effect of water within the hydration shell. On the other hand, in the high frequency region (1.0-1.8 THz), the spectra increased in amplitude as compared to that of pure water. This finding indicates dissolved ions weaken the hydrogen bonding of water in ionic solution and induce the structure breaking effect beyond the hydration shell. (C) 2013 Elsevier B. V. All rights reserved.