SUMMARY
This research aimed at comparing the effectiveness of Output-oriented and Input-oriented tasks on improving EFL learners’ vocabulary achievement. To reach their objective, the researchers ran a quasi-experimental pre-test posttest design with 64 Iranian EFL learners. After eliminating the initial differences among the participants, the Input-oriented group (IOG) received glossing tasks followed by selected reading passages, while the Output-oriented group (OOG) received gap-filling and composing/discussing tasks. Actively participating in 15 sessions of task-based vocabulary learning, both IOG and OOG performed on a vocabulary achievement test constructed and validated by the researchers (Cronbach a=.732). Despite the considerable improvement of the participants’ vocabulary knowledge, statistical findings failed to support the superiority of neither input nor output oriented tasks to make a meaningful difference in improving the Iranian EFL learners’ vocabulary achievement. Some implications and suggestions provided for further research.