This study presents a conceptual framework for providing intelligent supports through agent negotiation and fuzzy constraints to enhance the effectiveness of peer assessment. By using fuzzy constraints, it not only provides a flexible marking scheme to deal with the imprecision and uncertainty for the representation of assessment but also provides a computational framework to incorporate student's personal characteristics into the process for the reduction of assessment bias. Additionally, a fuzzy constraint-based negotiation mechanism is employed to coordinate the cognitive differences between students. Through iterative agent negotiation, students can reconcile the differences and reach an agreement on the assessment results. Thus, the proposed framework allows students to provide more detailed, informed, and less biased assessments for their peers' work. To demonstrate the usefulness and effectiveness of the proposed approach, a negotiation-based peer assessment system, NePAS, has been built and used in classroom. Experimental results suggested that students were more willing to accept the assessment results and able to acquire more useful information to reflect upon and revise their work. Instructors can also observe students' participation and performance to appropriately adjust instructional strategies. (Contains 4 tables and 11 figures.)