This paper aims to examine hedging expressions expressed by epistemic modal auxiliaries and to analyze their grammatical function, intrinsic meaning and scope in English. Meaning difference in sentences merged with an epistemic modal auxiliary is revealed by the scope parameter between the unmarked and marked interpretation of it. At the C-I interface, the unmarked use of an epistemic modal auxiliary doesn't contribute to the truth-conditional meaning, whereas the marked use of it sometimes contributes to the truthconditional meaning. An epistemic modal auxiliary moves to C from T at the C-I interface, then it functions subjectively as a discourse-related information marker connoting an illocutionary force feature there. But when it functions objectively at T without movement, an epistemic modal auxiliary links thematic relation to its subject. In order to confirm the scope of epistemic modals, 20 native speakers checked the grammaticality of sentences that containing both an epistemic modal auxiliary and a quantifier concurrently. The result shows that all the native speakers interpret the epistemic modal auxiliary as denoting wide scope, but they don’t agree one another on the interpretation of it as denoting narrow scope.