Abstract: The experimental Hugoniot data obtained by Mcqueen et al¹. show that there is a plateau on D-U plane. We think this is a first order phase transition from one solid to another solid which results in a more closely packed structure and decreased compressibility. The entropy change is calculated on the basis of Gibbs equlibrium conditions, using the equation of state for low pressures. The entropy change is negative, which means that this phase transition is a exothermic process. Then the equation of state for phase transition region is achieved, where the cold pressure is constant and the energy includes the effect of the phase transition entropy. The calculated Hugoniot using the above equation of state is in agreement with experimental Hugoniot presented by Mcqueen et al.