Upvotes serve important purposes in online social an-notation environments. However, limited studies haveexplored the influential factors affecting peer upvotingin online collaborative learning. In this study, we ana-lysed the factors influencing students' upvotes receivedfrom their peers as 91 participants utilized Perusall,an online social annotation system, for collaborativereading. The participants were asked to collaborativelyannotate 29 reading materials in a semester. We col-lected student reading behaviours and analysed theirannotations with a text-mining tool of Linguistic Inquiryand Word Count (LIWC). Moreover, conditional infer-ence tree was used to determine the relative impor-tance of explanatory factors to the upvotes studentsreceived. The results showed that the high-upvotegroup made significantly more annotations, postedmore responses to others' annotations and displayedfewer negative emotions in annotations than thosewho did not receive upvotes. The two groups of stu-dents had no significant differences in the upvotesgiven to others, as well as cognitive activities and posi-tive emotions involved in annotations. Moreover, thenumber of annotations was the determining factor inpredicting the upvotes that one could receive in socialannotation activities. This study has significant prac-tical implications regarding providing interventions insocial annotation-based collaborative reading.