This study investigates the effect of machine translation (MT) use on the writing performance of Korean EFL students, focusing on complexity, accuracy, and fluency (CAF). Six participants completed a series of writing tasks in which they first translated their L1 writing into L2 manually and then used MT to revise their L2 drafts. This process was repeated across ten different writing topics. Participants’ drafts were analyzed using CAF measures to assess MT’s impact on their writing performance and observe changes over tasks. The results show that MT significantly improved accuracy and fluency. However, gains in syntactic and lexical complexity were less evident. While group-level analysis showed consistent progress, individual trajectories varied widely, indicating diverse patterns of development. Overall, the findings suggest that MT enhances writing accuracy and fluency among Korean EFL students, although its impact on syntactic and lexical complexity is limited. These results indicate that MT can serve as a valuable tool for self-directed learning, helping students independently improve their writing accuracy and fluency and develop essential self-editing skills. This study highlights the potential of MT as a supplementary tool to support EFL students’ writing development, along with traditional instruction.

Abstract
1. INTRODUCTION
2. REVIEW OF LITERATURE
    2.1. Application of MT to Language Learning
    2.2. The Effect of Using MT in L2 Writing
    2.3. Complexity, Accuracy, and Fluency as QuantitativeMeasures of Writing Development in MT-Assisted Contexts
    2.4. The Effect of MT on Language Learners’ Writing Development
3. METHODOLOGY
    3.1. Research Questions
    3.2. Participants
    3.3. Writing Tasks
    3.4. Data Collection
    3.5. Data Analysis and Visualization
4. RESULTS
    4.1. Complexity, Accuracy, and Fluency Variation betweenOriginal Versions and Revised Versions
    4.2. Inter-individual Variation of CAF over Time
    4.3. Intra-individual Variation of Linguistic Measures over Time
5. CONCLUSION
REFERENCES
APPENDIX ATopics of Each Writing Task
APPENDIX BZ-Scores of S2 and S4

• Taeyeon Kim(Lecturer, Department of English Language and Literature, College of Humanities & Social Science, Hoseo University, 12 Hoseodae-gil, Dongnam-gu, Cheonan 31066, Korea)
• Sunmee Chang(Professor, Department of English Language and Literature, College of Humanities & Social Science, Hoseo University, 12 Hoseodae-gil, Dongnam-gu, Cheonan 31066, Korea) Corresponding author