The phase states of DNA in concentrated solutions have been studied by simultaneous measurement using differential thermal analysis (DTA) and a visible optical method (He-Ne gas laser, lambda(o) = 633 nm) equipped with a computer, and using a polarization microscope equipped with a hot stage, and IR spectra. From the polarization microscope results, DNA below a concentration of 2.00 wt% showed an isotropic phase; in the concentration range 3.00-5.00 wt%, DNA showed an anisotropic phase; and in concentrations above 6.00 wt%, DNA showed an anisotropic phase with a distinct ; birefringence from the measurement of the refractive index at 298 K, i.e. at concentrations above 6.00 wt% DNA forms a liquid crystal phase state. In order to obtain information about the mechanism of formation of the liquid crystal of DNA, IR spectra for solutions containing 10.0 wt% of DNA and Mg2+ ion have been measured at room temperature. DNA does not form liquid crystals in the presence of Mg2+ ion. Therefore, an electrostatic interaction between the PO, group in the main chain of DNA and the Mg2+ ion plays an important role in blocking the formation of the liquid crystal. Thus, it is suggested that DNA in concentrated solutions forms liquid crystals by a molecular orientation arrangement based on the repulsive force between the negative PO4- groups in the main chain of DNA. The change in the enthalpy of formation of the liquid crystal of DNA has been estimated to be about -2.8 kJ mol(-1) of nucleotide, by considerating the change in the intensity of transmitted laser light, using the DTA apparatus equipped with a laser.