Peer assessment has been widely applied to actively engage students in learning to write. However, sometimes students resist peer assessment. This study explores reviewers' attitudes and other underlying factors that influence students' participation in online peer assessment. Participants were 234 Chinese undergraduates from two different academic backgrounds: engineering majors (n = 168) and English majors (n = 66). Gender, academic background and prior experience with peer assessment were all related to participation levels. Moreover, factor analyses revealed three attitudinal factors: (1) "positive attitude" (a general endorsement of the benefits of peer assessment), (2) "interpersonal negative" (concerns about the negative effects on interpersonal relationships), and (3) "procedural negative" (doubts about the procedural rationality of peer assessment). Among the attitudinal factors, "procedural negative" was negatively associated with participation, as expected. Interestingly, "interpersonal negative" was associated with greater participation, and "positive attitude" was associated with lower participation, in part because students worked hard on each review rather than doing many reviews superficially. Implications for instruction are discussed.