The purpose of this action research project was to investigate the impact of a mastery-based grading system on the mindsets of high school students. The mixed methods data collection for this project took place over the course of two years. The students were part of a high achieving and mostly high socio-economic status community. I collected scores on the students' assessments, I administered a pre- and post- mindset survey to measure how much the students' mindset scores changed, and I utilized self-reflection prompts to capture the students' beliefs about learning and their own abilities. The first cohort of students showed a significant increase in their mindset scores. The second cohort did not display a significant change in mindset scores. A Pearson r correlational test between several variables and the change in mindset score revealed that neither of the cohorts, in the aggregate, showed significant correlations between the variables. The female subgroup of Cohort 2 displayed a weak negative correlation between the change in percentage of retakes completed and change in mindset score, while the Integrated Math (IM) 1 student subgroup and the IM 1female subgroup displayed moderate negative correlations between the percentage of second semester retakes completed and the change in mindset score. Neither the quantitative data nor the qualitative data provided clarity as to the meaning of these correlations nor conclusive explanations as to the causes of the changes in mindset scores. Thus, it is unclear if a mastery-based grading system has an impact on student mindsets in a high school mathematics class. In light of the findings, further cycles of action research could be conducted to investigate the correlations that were found. Additionally, including student interviews could illuminate connections between mastery-based grading and the changes in mindset scores that were evident. Another possible extension of this research could be to conduct case studies of students. A case study would allow for a deeper investigation into how mastery-based grading impacts an individual student which, when including interviews, could allow for a fuller picture of the relationship between mastery-based grading and student mindset. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]