Online discussion is one of the frequently used learning activities in an e-learning process. Learners can exchange opinions and thoughts with their peers through online discussion, stimulate their different viewpoints on the discussion topics, and enhance the learning effectiveness of e-learning. However, most learners mainly take an effort on posting discussion content but easily ignore reading and understanding the contents of peers' discussions, thus reducing the discussion effectiveness. The purpose of this study is to develop an Instant Perspective Comparison System with a Gamified Incentive Mechanism (IPCS-GIM) to promote learners' discussion awareness and motivation, thereby effectively enhancing their discussion participation, motivation, and effectiveness. A true-experimental design was conducted in this research to examine the research questions. A total of 38 university students in Taiwan were randomly assigned to the experimental group using the IPCS-GIM to assist online discussion and the control group using an instant perspective comparison system without a gamified incentive mechanism (IPCS-NGIM) to assist online discussion to examine their difference in discussion effectiveness, participation, and motivation on a social-scientific issue (SSI). The analytical results show that compared to the IPCS-NGIM, the IPCS-GIM could not only significantly enhance learners' discussion effectiveness in overall discussion effectiveness, complexity, and multiple perspectives, but also significantly enhance learners' discussion participation and motivation.