In recent years, there has been extensive research on serious games for edu-cational purpose. However, the design space for collaboration in games re-mains substantially unexplored. In this study, we systematically reviewed 31 empirical research articles regarding game-based collaborative learning published from 2006 to 2020 and attempted to provide new information about designing serious games for collaborative learning. We surveyed a number of games and investigated their design features that encourage col-laborative learning. Twenty game mechanics were identified and grouped in-to six main domains: (1) Space, (2) Objects, attributes and states, (3) Ac-tions, (4) Rules and goals, (5) Skill, (6) chance. The analysis of user studies they performed indicated that most of the game projects relied on self-report methods to test their learning effectiveness, and only a few studies adopted the data mining method based on game logs. The implications for research into facilitating collaborative learning and recommendations for future re-search directions are discussed.