Revision with unchanged content. Although considerable attention has been directed toward the value of peer assessment where learning is enhanced through maximizing the opportunities for students to share their work with others in the assessment process, very little work has been done to investigate the effects of the different types of learner role in peer assessment on learning. Therefore, this study investigated the effects of the assessor and assessee's roles on learners' metacognitive awareness, performance, and attitude. A particular focus is set on peer assessment as a leraning method. The analysis should help to answer the questions: - What are the comperative effects of the assessor's role and assessee's role on metacognitive awareness, performance, and attitude? -Does the role combination improve learners' metacognitive awareness, performance, and attitude to a greateer degree than either of the roles alone or neither role? -Are there any relationships between the quality of peer feedback and assessees' metacognitive awareness, performance, and attitude? The findings of the book suggest instructional implications for those who want to employ peer assessment as a learning method.