An ionic liquid domain was successfully prepared outside double-stranded DNA by fixing 1-alkyl-3-methyl-imidazolium (C(n)MI) cations on the phosphate groups of DNA. First, four species of ionic liquid were made using phosphoric acid di-n-butyl ester and C(n)MI (n=2,4,8, and 12) as a low molecular weight model. They were obtained as liquid salts, and their ionic conductivity ranged up to 10(-5)Scm(-1) at 50 degrees C. Based on this model study, counter cations of the phosphate groups of DNA were exchanged for four kinds of imidazolium cations. The resulting ionic liquid-robed DNA (IL-robed DNA) was soluble in ordinary organic solvents such as methanol or ethanol. Ionic conductivity was low, because the ion density was insufficient to form a continuous ionic liquid domain around the DNA strands. When 11mol% 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF(4)), which is a typical ionic liquid, was mixed with the IL-robed DNA, an ionic conductivity of 5.4 x 10(-5) S cm(-1) at 30 degrees C was observed because a continuous ionic liquid domain was successfully formed.