In this paper, a web-based peer-assessment approach is proposed for conducting performing arts activities. A peer-assessment system was implemented and applied to a junior high school performing arts course to evaluate the effectiveness of the proposed approach. A total of 163 junior high students were assigned to an experimental group and a control group. The students in the experimental group learned with the web-based peer-assessment approach, whereas those in the control group learned with a web-based streaming video-supported environment. The experimental results showed that, in comparison with the web-based streaming video-supported learning approach, the web-based peer assessment approach could significantly improve the students' performance, self-efficacy and motivation in the performing arts course. In the meantime, it was found that the peer assessment ratings were highly correlated with the teachers' ratings in every performance item; moreover, the performance ratings were highly related to the students' self-efficacy in evaluating peers' work and improving their own work based on peers' comments as well as their intrinsic motivation, showing the effectiveness of the performance scoring rubrics and the peer assessment approach. The satisfaction questionnaire results also revealed that the students who learned with the peer assessment approach were significantly more satisfied with the learning activity than those who learned with the web-based learning approach.