Nowadays online shopping is widespread. Consumer behaviors are affected by attractive product pages in online stores. However, there are a number of marketers providing irrelevant evidence for advertising claims like fancy diagrams or temping stories to mislead online shoppers. In particular, health food often claims to be good for the human body, but it may not be able to express relevant evidence nor clear information to support such a statement. This study uses two experiments to probe the interaction of the evidence (high vs. low relevance to health claims and clear vs. unclear information) and consumers’ product knowledge. The results found that consumers who have high product knowledge are more concerned about evidence whereas those having low product knowledge are not sensitive about how evidence is presented. Moreover, the interaction effect of the evidence and product knowledge has a significant effect on consumers’ evaluation through persuasion knowledge as mediation. In conclusion, this study suggests that marketers should express evidence that is highly relevant to advertising claims and provide clear information to enhance consumers’ overall evaluation of health food products.