From the viewpoint of consumer psychology, we argue that a key factor of co-branding success is a better product-fit in terms of attribute-level complementarity. The objective of this study is to investigate the impacts of attribute-level complementarity on co-branding success. We first define co-branding success, among others, as the occurrence of two effects, namely a synergy effect and a positive spillover effect. Then, assuming attribute-level complementarity exists in a horizontal co-branding setting, we test two hypotheses corresponding to each effect by using a perceptual Consumer-based Brand Equity (CBBE) measure in a laboratory experiment. The experimental results report that attribute-level complementarity may not lead to a “higher-value” co-branded product and may damage both brands’ equity. That is, the synergy and positive spillover effects may not always occur even under the scenario of a better product-fit. In sum, we find that a horizontal co-branding partnership with attribute-level complementarity could be a double-edged sword for the allying brands. The present paper contributes to co-branding literature by showing the connection between affect-transfer of attribute beliefs and co-branding success. For brand managers, our CBBE measure provides an ex-ante evaluation of a proposed partnership.