Positive student attitude towards peer assessment can be demonstrated through high-quality peer assessment activities. Research findings on students' attitudes before and after such activities were mixed. This study was conducted using a mixed-methods approach to investigate the underlying factors influencing students' attitude change. Participants were students enrolled in an online graduate-level assessment course in the college of education in a university in the southeast United States in the fall of 2017 (N = 31). Surveys and interviews were used to collect quantitative and qualitative data separately, both of which indicated that perceived accurate and specific feedback tended to help students shift to more positive attitudes towards online peer assessment. In addition, final task score improvement after the activity was correlated with positive attitude change. Qualitative results also identified other factors, such as communication with the peers' work and logistic concerns. The current study offers insights for researchers and instructors to promote positive attitude changes in online peer assessment activities.