With the proliferation of hedonic information systems, understanding users' acceptance of hedonic information systems has become a new topic for practitioners and academics. While perceived playfulness or perceived enjoyment has been found to have a significant influence on the behavioral intention to use hedonic information systems, little research has been conducted to investigate empirically the antecedents of perceived playfulness and the mediating role that perceived playfulness has in user acceptance of hedonic information systems. Thus, the main purpose of this study is to explore the mediating role of perceived playfulness in the psychological process of user acceptance of hedonic online game systems. Based on previous literature, two individual difference variables (i.e., computer self-efficacy and computer anxiety) and three system characteristics variables (i.e., challenge, feedback, and speed) were proposed as potential antecedents of perceived playfulness in the context of massive multiplayer online games. The results indicate that perceived playfulness plays a partial mediating role in the relationship of system characteristics and individual differences to behavioral intention. Both challenge and computer self-efficacy were found to have a significant influence on behavioral intention via perceived playfulness, with computer self-efficacy also having a direct influence on behavioral intention. Computer anxiety, however, was only found to have a direct influence on behavioral intention. Also, neither feedback nor speed was found to have a significant effect on perceived playfulness. The results of this study provide several important implications for research and practices of hedonic information systems/online game design and promotion.