Dielectric relaxation measurements on water solutions of ethylene glycol 200 and 400, (degree of polymerization N = 4 and 9) in entire concentration region were carried out using a time domain reflectometry at 25 degrees C in the frequency range from 300 MHz to 20 GHz. For all the samples, only one dielectric loss peak was observed in this frequency range, Plots of the relaxation strength and logarithm of the relaxation time calculated from apparent peak frequency of dielectric loss curves against monomer unit molar fraction of ethylene glycol X give straight lines in the region of 0 < X < 0.35 for N = 4, and 0 < X < 0.37 for N = 9, Shapes of dispersion and absorption curves exhibit critical change at the concentration X approximate to 0.35 for N = 4 and X approximate to 0.37 for N = 9, corresponding to the ratio of one ether oxygen and 1.7 water. Analysis of these phenomena indicates that hydration complex of one ether oxygen and 1.7 water is formed, and the 1:1.7 complex behaves as one kind of component corresponding to 2.7(= 1 + 1.7) waterlike molecules in the solution, It is suggested that ether oxygen can be inserted into water structure by replacing water oxygen. This hydration mechanism makes water structure stable. Ethylene glycol dissolves in water without much perturbation to water structure.