Using the Game-based-learning Teaching Belief Scale (GTBS) and the Technological Pedagogical Content Knowledge--Games questionnaire (TPACK-G), this study investigated 316 Taiwanese in-service teachers' teaching beliefs about game-based learning and their perceptions of game-based pedagogical content knowledge (GPCK). Both t-tests and ANOVA analyses were used to examine the differences due to their demographic backgrounds. Correlation and regression analyses were used to examine the predicting variables of the teachers' GPCK. The results showed that, among the factors of GTBS and TPACK-G, GPK plays the most critical role in predicting GPCK. In addition, the elementary school teachers had stronger Belief, Confidence and Motivation for using game-based teaching approaches, and had higher GPK and GPCK than middle school teachers. A gender difference only existed in GK. Younger teachers had better GK and GCK than older teachers. Compared with experienced teachers, novice teachers tended to believe that digital games can assist learning and instruction, and perceived higher self-efficacy in their TPACK-G.