Although Massive Open Online Courses (MOOCs) attract millions of people to enroll in their courses, the completion rate for most courses is very low. A majority of MOOCs students are not fully engaged in MOOCs, thus leading them to quit in the early stage of the courses. Therefore, it is important to investigate students' engagement in MOOCs. Drawing on self-determination theory and the theory of relationship quality, this study proposes a model that conceptualizes the MOOCs engagement as consisting of psychological engagement and behavioral engagement and explores the antecedents of students' engagement in MOOCs. The research model is tested using data collected from 374 students of Chinese University MOOC. The results show that fulfillment of three basic psychological needs for autonomy, competence and relatedness have significant positive effects on intrinsic motivation, increasing students' psychological engagement in MOOCs. Relationship quality significantly predicts students' psychological engagement, and psychological engagement promotes behavioral engagement in MOOCs. Implications for research and practice as well as limitations of this study are discussed finally.