Abstract: Over the past three decades, understanding of metaphor has undergone a fundamental transformation:Metaphor is not only regarded as a language phenomenon, but also an important cognitive mechanism.Metaphor can affect participants in communication on the cognitive level, and influence their behaviors.As one of the most important theories of argumentation, Pragma-dialectics has already noticed the use of metaphor in argumentation, but most pragma-dialecticians regard it as a strategic maneuvering from the aspect of presentational device, ignoring metaphor's cognitive function.The present study, from a Pragma-dialectical perspective, draws insights from framing effect of metaphor and explores the strategic function of framing by metaphor in argumentation on all three aspects of strategic maneuvering.Metaphor helps enhance argumentative effectiveness by foregrounding concepts, expressing in euphemism and consolidating memory.In addition, the reasonableness of framing by metaphor is discussed, together with its effectiveness and reasonableness in argumentation.