This paper reports a study on the use of summary writing as an assessment tool in content-based instruction in the EFL context. The participants were 50 English majors from a Chinese University. In the 16-week teaching of the content course A Concise History of Western Civilization, the teacher researcher collected data through summary writing, reflective journal writing, and questionnaire survey to explore the instructional effects of summary writing and students’ views on summary writing as a tool of assessment. By analyzing chapter summaries submitted regularly, the teacher researcher was able to assess students' mastery of the content and improvement made in writing. By analyzing reflective journals and questionnaire responses, the teacher researcher got an idea of students' reflections on the learning process and their perceptions of summary writing. Data analysis showed that summary writing deepened students’ understanding of chapter content, improved their writing accuracy and fluency, and strengthened their awareness of language imitation in content-based instruction. It also revealed that students generally showed a positive attitude toward summary writing. Pedagogical implications and suggestions are also discussed.