This study developed an educational online game, Super Delivery, targeting knowledge about saving electricity, and conducted case studies of eight sixth-grade students using this game to explore the factors influencing the effectiveness of students' knowledge acquisition in digital game-based learning (DGBL). This study followed Miles and Huberman's (1994) suggested procedure to analyze qualitative data and find the patterns across cases. Based on the analysis of the case-ordered predictor-outcome matrix and diverse evidence including qualitative and quantitative data, a casual map and a decision tree were constructed to explain why differences in the effectiveness of knowledge acquisition existed among the study participants. It was found that many factors interactively influence students' effectiveness of knowledge acquisition in DGBL. Students' learning motivation, learning ability, and playing skill could be key factors that collectively influence the effectiveness of knowledge acquisition in DGBL. Also, students' learning motivation, learning ability, and playing skill were affected by their playing motivation, prior knowledge, as well as online game experience respectively. The results of this study may help teachers consider how effectively utilizing an educational game for enhancing students' learning effectiveness in DGBL. (Contains 1 table and 4 figures.)