This study expands the traditional TAM by considering two mediators between innovation characteristics and the adoption of online payment services: perceived use efficiency and perceived use effectiveness. It also proposes the influential paths from innovation design characteristics to innovation adoption through user perceptions and experience in order to better understand how to design an acceptable innovation and identify the targeted consumer segments. This study developed experimental websites and simulated innovation characteristics to test the hypotheses. The results indicate significant mediating effects of perceived use efficiency and perceived use effectiveness on the relationship between innovation characteristics of the online payment websites and the adoption intention to use the new service. These results of the experiments support the expanded conceptual framework for TAM with two pathways-one innovation characteristics-based and the other user perceptions-based-connecting to the acceptance of new technologies. Collectively, for predicting the adoption of online payment services, the mediating model is more effective than the direct effect model of innovation characteristics in a traditional TAM.