As a major instrument of quality assurance implemented in online business, quality certifications play pivotal roles in retailing platforms’ operations. This paper studies whether retailing platforms can improve the product quality of sellers (manufacturers) through implementing quality certifications. We develop a game-theoretic model which consists of one retailing platform and two competing manufacturers with differentiated brand positioning. Considering the platform can flexibly set the quality standard, we explore the quality certification for the platform where the quality and price decisions of the heterogeneous manufacturers can be affected. We find that the implementation of the certification is not always profitable for the platform. Specifically, when the certification cost coefficient is low (high), the certification is profitable (unprofitable) for platform. Moreover, when implementing the certification, the platform may prefer to only encourage the high-end manufacturer to participate in the certification, as the participation of the low-end manufacturer could be unprofitable for the platform. More interestingly, we find that the effects of the introduction of quality certification are two-fold. It enhances the price and quality of the participant while incentivizing the non-participant to degrade its product price and quality. Thus, the certification expands the vertical differential of the products sold via the platform. Finally, the robustness of the results established in the main model is validated in other extensions including linear certification costs, the impact of platform commission fees and unit traveling cost.