The current study investigates EFL college learners' motivation and engagement during English vocabulary learning tasks. By adopting self-determination theory (SDT; Deci & Ryan, 1985, 2000), the study looked into the impact of autonomy on college students' task motivation and engagement with vocabulary learning tasks and their general English learning motivation (i.e. trait motivation). Changes in task motivation over 14 weeks of research were also investigated. The results of quantitative analyses obtained from questionnaires and observations over 14 weeks uncovered that the participants who were given the freedom to choose their own target words showed higher task motivation and task engagement than those who were required to learn pre-selected target words. Learners' general attitude toward English learning, however, was not influenced by the provision of choice in the vocabulary learning tasks. The 14-week experimental research also depicts the changes of task motivation and engagement over time. Even with the same settings and task procedures, learners' task motivation fluctuated. In accordance with SDT, the findings in the present study support the importance of creating autonomy-supportive vocabulary learning tasks.