The present study, using quantitative and qualitative analyses, aimed at delineating the interrelationship between the knowledge of metacognition and the regulation of metacognition, along with the role of learners' regulatory ability in mediating the effects of task-induced involvement load on word learning. A total of 60 university EFL students were recruited to the study. They first completed a checklist on metacognition and were then assigned to complete three tasks with varying degrees of involvement load followed by a vocabulary test. Of them, 12 students also participated in an interview. The results showed that the two main components of metacognition, i.e., the knowledge and regulation of metacognition, are closely and significantly correlated. The learners, assigned to four different ability groups (LK/LR, LK/HR, HK/LR, HK/HR), were found to benefit most by engaging in a task with the highest involvement load. Despite the benefits, their regulatory ability mediated the effects of task-induced involvement load on word learning, which was corroborated by the interview results. The relevant implications for teaching and learning words through tasks are further discussed.