With the advancements in technology networks, mobile learning has been extensively applied in teaching contexts, enabling closer interaction between students and authentic environments. Previous studies have indicated that, when learning in real-world contexts, students need to interact with real learning targets and peers as well as the digital learning content; thus, effective learning strategies or tools are generally required for helping them organize knowledge and for promoting peer interactions. As a result, the present study developed a bi-directional peer-assessment-based concept mapping system, which not only engages students in assessing peers' concept maps, but also encourages them to evaluate or respond to peers' ratings and comments. To explore the effectiveness of this learning system, the study conducted an experiment in elementary science classes. A total of 101 fifth graders from four classes were recruited to participate in the study. Two classes were assigned to the experimental group and two to the control group. The experimental group adopted the bi-directional peer-assessment approach, while the control group adopted the conventional peer assessment approach for concept mapping. The results reveal that the bi-directional peer-assessment-based concept mapping system can not only increase students' learning achievement and the completeness of their concept mapping, but can also enhance their critical thinking tendency and the feedback quality of their peer assessment.