In the social interactions among individuals of learning communities, including those individuals engaged in peer assessment activities, emotion may be a key factor in learning. However, research regarding the emotional response of learners in online peer assessment activities is relatively scarce. Detecting learners' emotion when they make comments and responses to peers may be helpful to better understand their learning and participation behaviours. Therefore, the aim of this study is to explore the emotional responses and participation of university students in online peer assessment. The self-assessment manikin (SAM) (Lang, P.J. (1985). The cognitive psychophysiology of emotion: Fear and anxiety, In A.H. Tuma & J.D. Maser (Eds.), "Anxiety and the anxiety disorder" (pp. 131--170). Hillsdale, NJ: Lawrence Erlbaum.) measurement was integrated into the online peer assessment system to measure learners' emotional responses, and a cluster analysis was implemented to identify various features of the different groups. The results show that response behaviours may reflect a learner's participation and status of emotional response during an online peer assessment process. Using cluster analysis, the groups were divided into three clusters; each cluster's representation of emotional response patterns was then analysed and discussed. Finally, this study proposed technical mechanisms in online peer assessment systems (i.e. data mining or RSS feeds) for improving learners' participation and supporting teachers or researchers to intervene to provide relevant assistance and guidance when necessary.