In this paper, we deal with how agents reason about knowledgeof others in multiagent system. We first present a knowledge representation frame-work called reasoning about others (RAO) which is designed specifically to representconcepts and rules used in reasoning about knowledge of others. From a class ofsentences usually taken by people in daily life to reason about others, a rule calledposition exchange principle (PEP) is abstracted. PEP is described as an axiomscheme in RAO and regarded as a basic rule for agents to reason about others, andfurther it has the similar form and role to modus ponens and (K) axiom of knowl-edge logic. The relationship between speech acts and common sense is also discussedwhich is necessary for RAO. Based on ideas from situation calculus, this relationshipis characterized by an axiom schema in RAO. Our theories are also demonstrated byan example. In this paper, we deal with how agents reason about about knowledge of others (RAO) which is designed specifically to represent con- fances and rules used in reasoning about knowledge of others. From a class ofsentences usually taken by people in daily life to reason about others, a rule calledposition exchange principle (PEP) is abstracted. PEP is described as an axiomscheme in RAO and regarded as a basic rule for agents to reason about others, andfurther it has the relationship between speech acts and common sense is also discussedwhich is necessary for RAO. Based on ideas from situation calculus, this relationshipis characterized by an axiom schema in RAO. Our theories are also demonstrated by example.