While research on postgraduate thesis writing has investigated how students cope with institutional assessment criteria, this study explores how students form their own criteria for self-assessment through the writing process, aiming to account for the development of their independent thinking in academic socialisation. Based on in-depth interviews with 10 Chinese master's students, the study found that their self-assessment criteria not only represented solutions to problems identified by their supervisors and examiners, but also embodied aims established by students themselves. Those criteria were not fixed textual properties of a 'good' thesis, but were dynamic representations of textual and social realities constituting students' lifeworlds. Viewed from this perspective, self-assessment is a situated process whereby students re-experience and re-interpret multiple sets of realities juxtaposed against their writing. Pedagogically, the study implies that postgraduate thesis supervision could facilitate the expansion of students' self-assessment criteria to develop their capacity for making independent judgements.