Elaboration Likelihood Model (ELM) which explained the consumer’s attitude formation process has been widely used by the marketing industry since 1980s. In this study, we define argument diagnosticity of strong, weak and neutral argument. To further explore the relationship between argument quality and argument diagnosticity. In this study, we found that high argument diagnosticity did assist high involvement customers to differentiate the quality of different brand’s products. And if the argument diagnosticity is low, then high involvement customers are unable to distinguish the product’s quality through this argument and the attitude toward the product is unfavorable. Keywords: elaboration likelihood model, argument quality, argument diagnosticity, correction