The purposes of this study were to explore the effects and the validity of on-line peer assessment in high schools and to analyze effects of various types of peer feedback on students. The participating students, a total of 184 10th graders, developed their individual course projects by involving the on-line peer assessment learning activities in a computer course. The peer assessment activities consisted of three rounds, and each of the students acted as an author and a reviewer. Research data as evaluated from peers and experts indicated that students significantly improved their projects as involving the peer assessment activities. The scores determined by the learning peers were highly correlated with those marked by the experts, indicating that peer assessment in high school could be perceived as a valid assessment method. Moreover, this study also examined the relationships between the types of peer feedbacks in which students obtained from peer assessment and the subsequent performance of their projects. We categorized peer feedbacks into four types: Reinforcing, Didactic, Corrective and Suggestive. It was found that Reinforcing peer feedback was useful in helping students’ development of better projects; however, Didactic feedback and perhaps Corrective feedback provided by peers might play an unfavorable role for subsequent improvement of students’ projects. The Suggestive feedback may be helpful in the beginning of peer assessment activities; however, in the later parts of peer assessment, the effect of this type of feedback on learning might not be significant.