Abstract: There exist problems in the two concepts of monetary nature in western economics, which can hardly explain the monetary phenomenon and economic reality. Marx once criticized the both in his works. His duality analytical method provides an ideological weapon for objectively recognizing the two concepts of the nature of money and comprehensively understanding the multi-dimensional connotation of money. Marx's duality thought holds that money essentially has two properties: social attribute and natural attribute, which is a commodity that acts as a general equivalent. Based on the method of the unity of history and logic, Marx advocated the idea of rising from abstraction to concrete analysis. The historical evolution of the social form of money provides a historical basis for Marx's monetary analysis. Marx's duality method is also applicable to the analysis of monetary functions. The general equivalent can extend two basic functions: value scale and transaction media, both can be interpreted in duality. Based on the overlapping of monetary functions, the construction idea of monetary theory can be obtained.

Key words: duality thought, monetary theory, commodity essence, credit essence, social form