Peer assessment is an indispensable part in classroom assessment and it serves as a very useful way of promoting learning. However, different ways of peer grouping may influence the validity of peer assessment. This study analyzes the quality and quantity of feedback, adoption rate of feedback as well as scores of students' original drafts and the revised versions. It finds that all ways of grouping can promote learning but the degree of validity varies among groups. Besides, accuracy and adoption rate are high in students' feedback, which means peer feedback is effective to a great extent. Among all the ways of grouping, homogeneous grouping i.e. pairing students with the same or similar language proficiency level can archive more significant promotion in learning. In general, students hold a positive view towards the validity of peer assessment.