In the field of teaching English as a Second or Foreign Language, researchers have addressed the importance of teachers’ language use on the premise that English as a medium of instruction will accelerate students’ language acquisition process as well as product. One of the symbolic examples which reflect on this popular belief could be English Medium Instruction (EMI). However, the efficacy of this instructional strategy still needs to be further scrutinized to provide solid empirical evidence. With the combination of EMI and written corrective feedback in writing, this study used quantitative approach to test the efficacy of these combined instructional strategies and qualitative approaches to depict how EMI and Korean-led students' acquisition paths are similar or different in terms of the acquisition of global errors. The results of the study indicate that EMI students outperformed Korean-led students in short and long term period of writing tasks. As to acquisition paths, when global errors are a primary concern, EMI exposed students also show less errors at the final stage of the instruction which differentiates them from Korean-led students. This study provides empirical evidence supporting the positive role of medium of instruction in second language writers’ writing development. Based upon the findings of the study, suggestions on what to teach first and next to a specific group of learners, for this case 69 out of 100 score, are provided.