This study examined the impact of online peer assessment based on descriptive questions on student learning in Korea. The validity of the scores for student-written descriptive responses rated by middle and high school students were compared to teacher grading. The impact of peer assessment activities based on descriptive questions on student learning was also investigated, as was investigated students' perceptions of peer assessment. First-year middle and high school students studying science were recruited. The results reveal a high level of agreement between peer assessment scores of middle and high school students and teacher grades. The peer assessment activity based on descriptive questions enhanced students' academic achievement. Students stated peer assessment activity was effective for conceptual learning. This study confirmed the effectiveness of peer assessment activities based on descriptive questions in students' conceptual learning.