Following the increasing use of online learning among academic institutions worldwide, this study examined the relationships between enjoyment of study, study engagement, learning performance, and informal learning. Based on the intrinsic motivation theory, enjoyment of study is posited to motivate students to engage in their studies and affects their learning performance. In this study, study engagement was used as a mediator between enjoyment of study, learning performance, and informal learning. Data were collected from 137 undergraduate students in a Hong Kong university who participated in online learning for an academic semester. The results indicated that enjoyment of study and study engagement were "positively" related to learning performance and informal learning. Study engagement "partially" and "fully" mediated the relationships of enjoyment of study with learning performance and informal learning, respectively. Theoretical and practical implications for enjoyment of study in online learning are discussed.