This study attempted to delineate the relationship between the knowledge and regulation of metacognition, and to show how they interact to mediate the effects of task-induced involvement load on learning vocabulary. A total of 90 tertiary-level students completed a checklist on metacognition. Subsequently, they were assigned to complete three tasks with varying degree of involvement load and to complete certain vocabulary tests. Results showed that both the knowledge and regulation of metacognition are independent constructs, but closely and significantly correlated. The learners were sub-divided into two distinct ability groups (high vs. low) based on the knowledge-of-metacognition checklist, and two distinct ability group (high vs. low) based on the regulation-of-metacognition checklist. Overall, the learners were divided into four groups: (1) low knowledge/ low regulators; (2) low knowledge/high regulators; (3) high knowledge/low regulators; and (4) high knowledge/high regulators. Learners benefited the most by engaging in a task with the highest load of involvement. However, learners with a high level of regulation of metacognition performed well in the three tasks, which suggests a mediating role of learners' regulatory ability. Relevant implications were discussed on how to effectively apply task-induced involvement load into learning new words from the perspective of metacognition.