The peculiar chemical properties of nanoparticles are quite different from their bulk counterparts, the differences are related to their surface thermodynamic quantities, mainly depending on their particle sizes. In this paper, the relations between solid-gas surface thermodynamic quantities and particle size at constant temperature and constant volume were derived. And the theory of decomposition thermodynamics at constant temperature and constant volume was proposed and the relations of equilibrium constants and thermodynamic properties of decomposition reactions with particle size were derived respectively. Applying the theory, the method of obtaining the surface thermodynamic quantities by thermal decomposition was proposed. Experimentally, the thermal decomposition of nano-Cu-2(OH)(2)CO3 with different particle sizes was taken as a research system, the standard equilibrium constants, the thermodynamic properties and the heat capacity difference for decomposition of nano-Cu-2(OH)(2)CO3 with different sizes were obtained by calculating the partial pressure and the fugacity coefficient of each gas component by RK equation at different temperatures. Further, the solid-gas surface thermodynamic properties, surface tension and surface heat capacity were obtained. And the size dependence of surface thermodynamic quantities and decomposition thermodynamic quantities of nano-Cu-2(OH)(2)CO3 are consistent with the theoretical relations above.