This paper is devoted to the study of thermodynamic properties on the Berthelot fluid. The Helmholtz

free energy，entropy，Gibbs function，internal energy and enthalpy of monoatomic Berthelot fluid are derived;the

parametric solutions of the Berthelot fluid two coexisting phases are obtained，and phase diagrams，thermodynamic

behaviors near the critical point and associated properties are discussed by using the parametric solutions. The study

provides analytic theory of thermodynamics and phase transition for substances satisfying the Berthelot fluid.