Each year businesses spend millions of dollars on advertising to engender a positive attitude among consumers about a specific brand, eliminate any mistrust and suspicion about advertising, and rectify wrong attributions about products. In this research we focused on how doubt about advertisements impacts on their effectiveness, in regard to brand attitude and purchasing intention. We adopted the seemingly unrelated regression model to estimate the parameter, and the mutual relationships among several regression equations are taken into consideration. There were 337 participants in this experiment, and as indicated by the result, doubt about advertisements had a negative effect on brand attitude and purchasing intention. This negative effect will disrupt product involvement.