Abstract: Microeconomics refers to a set of principles and assumptions be observed when studying social phenomena as a method of thinking. The first principle of microeconomics is positivist scientism, which divides microeconomics into positive economics and normative economics. Positive economics is based on two basic assumptions, namely, methodological individualism and economic rationality, which jointly generate the second principle —the economic rationalism, and derive two analysis tools: optimization analysis and equilibrium analysis. The third principle of microeconomics is utilitarianism, from which welfare economics (normative economics) originates, and an evaluation index derives—Pareto Efficiency or economic efficiency. These principles, assumptions, analysis tools and evaluation indicators make up an internal logical relationship.