The present study was primarily aimed at investigating how Processing instruction would affect the Iranian EFL learners’ grammatical knowledge and how the effectiveness of this method would be modulated by the learners’ individual differences in grammatical sensitivity. To this end, three senior intact high school classes were selected and randomly assigned to two experimental and one control group. Each of the experimental groups was treated with one operationalization of Processing instruction, namely, full PI (n=24), and Structured input (n=24) while the control group (n=20) received traditional output-based instruction (TI) on the English passive structure over three weekly sessions. The results illustrated that Processing instruction was more effective than TI since it improved learners’ both interpretation and production while TI only could improve learners’ production. Additionally, the results demonstrated that the positive impact of Processing instruction was not affected by the learners’ differences in grammatical sensitivity. All in all, this suggests that as long as a grammatical structure is affected by an underlying processing problem, Processing instruction is possibly a more effective pedagogical option compared to traditional output-based instruction. Furthermore, Processing instruction seems to work for all learners irrespective of their differences in terms of grammatical sensitivity.
