Digital games have some traits, such as fantasy, rule, goal, excitement, mystery, challenge, and control. It makes people immerse in and concentrate on digital games, and that kind of experience can make people focus on learning. Yet, participants could only want to play digital games, but not want to learn from digital games. The reason is that the link between learning contents and game contents isn’t tight enough. Learning companions have characteristics such as peer interaction, gaining motivation, and learning outcomes. Therefore, this study constructs an online game-based learning system with learning companions to solve the problem in linking learning contents and game contents. In the experiment with 6th graders in an elementary school, we find that the achievement of the students using online game-based learning system with learning companions are higher than that of the students using online game-based learning system without learning companions.