This study adopts a case study approach to examine how students write in English-medium instruction contexts. It also explores why they write in this way from the perspective of writer identity. Four Chinese university students’ EMI course essays, as well as their interview and stimulated recall responses were collected. The analysis results presented three patterns of writer identity: (1) a member, as an EMI writer, of the academic community as the dominant self; (2) a student writer meeting the course requirements as the dominant self; (3) struggling between the two selves. Having different types of writer identities, the students wrote their EMI course essays in different ways. Their writings presented different features in terms of discoursal choice, language form and format. Suggestions for EMI teaching, EMI teacher training and curricula at the university level are provided.