Abstract: The microstructure of solution is analyzed using statistical thermodynamics. The result shows that a single solute molecule may cause the rearrangement of solvent molecules around the solute. The interaction between two solute molecules will lead to rearrangement of solvent molecules when the concentration of the solution is large enough. To describe the reorganization properties of finite concentration solution, the new concepts of second order reorganization energy and reorganization entropy, which can describe macroscopically the reorganization properties due to interaction between two solute molecules are proposed. Formulas of first and second order reorganization energy and reorganization entropy are obtained by using cluster expansion method. Relation be tween radial distribution function of a pair of solvent molecules and chemical potential of solute and solvent is de rived. The macroscopic criterion for determining infinite dilution is obtained by use of the relation. The theory of this paper can be used in the case of low density.