This study reports the effects of online peer assessment, in the form of peer grading and peer feedback, on students' learning. One hundred and eighty one high school students engaged in peer assessment via an online system--iLap. The number of grade-giving and grade-receiving experiences was examined and the peer feedback was coded according to different cognitive and affective dimensions. The effects, on both assessors and assessees, were analyzed using multiple regression. The results indicate that the provision by student assessors of feedback that identified problems and gave suggestions was a significant predictor of the performance of the assessors themselves, and that positive affective feedback was related to the performance of assessees. However, peer grading behaviors were not a significant predictor of project performance. This study explains the benefits of online peer assessment in general and highlights the importance of specific types of feedback. Moreover, it expands our understanding of how peer assessment affects the different parties involved. (Contains 3 figures and 4 tables.)