The purpose of this study is to analyze online peer feedback provided during high school physics lab classes. A stage of online peer feedback was included, and thirteen high school students participated in the stage to provide online peer feedback on the experiment report written during a physics lab class. The students were asked to add a comment on the reports for feedback. Characteristics of online peer feedback have been analyzed, followed by analysis of responses of students who received feedback depending on addition of comments for feedback. Evaluation standards of experiment reports have been used to compare the reports between before and after receiving online peer feedback, and a survey has been conducted to examine students’ perceptions of online peer feedback. The results show that the most feedback was provided on the parts with unclear description, and most the comments included detailed and reasonable feedback criticizing the contents or suggesting more acceptable descriptions. Conclusively, online peer feedback has significantly increased student’s final marks of experiment reports at each stage, and students have found to positively perceive online peer feedback taken place during physics lab classes.