Many studies have verified that the effective promotion of both positive interactions among individual group members and group accountability is a critical issue in collaborative problem-based learning (CPBL). This work, therefore, proposes a group incentive mechanism (GIM) that is based on considering several important factors that influence peers' interactions and group accountability in collaborative learning to improve learning performance, interactive relationships, group efficacy, and the cohesiveness of groups of learners in a CPBL system. To evaluate the effectiveness of the proposed GIM, 48 Grade 4 students from two classes were recruited from an elementary school in Taoyuan City, Taiwan, to participate in an instruction experiment. Two classes were randomly assigned to the experimental group using CPBL with the proposed GIM and the control group using CPBL with the individual incentive mechanism (IIM) to solve a target problem collaboratively. Analytical results reveal that although the control group of learners with the IIM exhibited greater social interactions than the experimental group of learners with the proposed GIM, the experimental group exhibited better learning performance, group efficacy, and positive interactive relationships than the control group.