Mathematics learning plays a significant role for high school students. Although gender differences in mathematics learning have been well documented, gender differences in mathematics learning self-assessment have not been adequately studied. The purpose of this study was to investigate whether gender differences existed in the mathematics learning self-assessment process. In this study, nine Chinese high school students answered interview questions about self-assessment (Step 1: determine/set self-assessment criteria; Step 2: seek external and internal feedback; Step 3: reflect the learning process and product with the support of feedback) in mathematics learning. The findings revealed that all high school students self-assessed their mathematics learning. Gender differences varied across the self-assessment steps. In Step 1 and Step 2, it did not show significant gender differences. However, in Step 3, gender differences were significant. This study can help educators and researchers better understand the variability and complexity of self-assessment in mathematics learning. [For the full proceedings, see ED631133.]