This paper examines how existing knowledge base (i.e., knowledge breadth and depth) interacts with knowledge integration mechanisms (i.e., external market knowledge acquisition and internal knowledge sharing) to affect radical innovation. Survey data from high technology companies in China demonstrate that the effects of knowledge breadth and depth are contingent on market knowledge acquisition and knowledge sharing in opposite ways. In particular, a firm with a broad knowledge base is more likely to achieve radical innovation in the presence of internal knowledge sharing rather than market knowledge acquisition. In contrast, a firm with a deep knowledge base is more capable of developing radical innovation through market knowledge acquisition rather than internal knowledge sharing.