Playfulness steering is an emerging approach in educational game design and play. The integration of arithmetical computation, game strategy, and teamwork into one game allows players to interactively "steer" the playfulness and enhance learning. In this paper an evolutionary contest game was designed and implemented to examine the influential factors. Using action research, focus groups and hermeneutic methods, this study suggests that the playfulness design of an evolutionary game is influenced by the degree of uncertainty and flexibility in decision-making, the level of challenge, equal conditions for fair play, opportunities to compete/cooperate, and the level of interactivity. (Contains 2 tables.)