Abstarct: In this study, thermodynamic properties of natural gas have been calculated using AGA8 Equation of State (EOS). Natural gas is a mixture of different gases with various properties, natural gas properties strongly influence properties of its component. Therefore, as composition of natural gas changing, natural gas thermodynamic properties are changed as well. In order to calculate the thermodynamic properties of natural gas, we need to have the equation of state to calculated thermodynamic properties of natural gas according to its components. AGA8 EOS presented by the America Gas Association, by considering the interaction properties of natural gas component in the gas mixture, has been offered very accurate models for calculating natural gas compression factor. In this research, using AGA8 EOS and using certain thermodynamic relations between the natural gas components in the gas mixture, natural gas thermodynamic properties have been calculated. Calculations have been validated with experimental data. Validations showed AGA8 EOS has high accuracy in calculation thermodynamic properties of natural gas. Thermodynamic properties calculated for different regions of Iran's gas fields and effect of natural gas compositions on the thermodynamic properties have been investigated. Also, effect of natural gas composition on these processes has been analysis. Finally, software for calculation thermodynamic properties of natural gas has been presented. With known of the temperature, pressure and mole fraction of natural gas components in the gas mixtures, the thermodynamic properties of natural gas can be calculated using designed software.