The purpose of this study is to analyze relationship between printed book and e-book formats based on niche theory. Readers with experience reading printed book and e-book were recruited as the research participants. In this study, competition between printed book and e-book formats was measured at gratification niche. five gratifications were derived from an analysis of precedence about printed book and e-book; information acquisition, social interaction & decision making, emotion & habit, portability & storage capacity, and accessibility. The results show that e-book formats have competitive superiority of portability & storage capacity and accessibility. This study suggests that it is possible to replace printed book and e-book formats.