While previous research has investigated student perceptions of the benefits of self-assessment in print-based L2 writing tasks, little research has delved into students' self-assessment processes in technology-enhanced L2 writing tasks, which may create new opportunities and challenges for self-assessment. Drawing on multiple sources of data, including student interviews, classroom observations, and documents, this multiple case study examines how Chinese university students of differing L2 proficiency assess their own digital multimodal composing (DMC) projects. The findings revealed that L2 student writers were involved in the processes of determining performance criteria, self-directed feedback seeking, self-reflection, and taking action in their DMC self-assessment. The findings also indicated that while the high-proficiency students' self-assessment process was more cyclical, the middle- and low-proficiency students’ self-assessment processes were more linear, and they also exhibited differing features in their specific DMC self-assessment processes. This study provides insights into conceptualizing models of student self-assessment in L2 writing and adds to our understanding of how to assess student performance in technology-enhanced L2 writing tasks.