Reputation management systems have been proposed to provide helpful information for reliable peer selection in an environment where honest peers coexist with malicious peers. The reputation management systems generally seek to generate an accurate assessment in the face of various factors including but not limited to potentially adversarial environments. Thus, performance of the reputation management systems mainly depends on an effectiveness of detection of various malicious attacks.In this paper, we focus on a detection of colluders who usually form a clique among various malicious attacks since only heuristic-based approaches which are still vulnerable to sophisticated colluding attacks have been proposed due to a NP-completeness of the detection of colluders forming a clique. For collusion-resistant reputation management, we introduce a simplified clique detection that can be applied to the reputation management system so that the colluders forming a clique can be easily detected. Then, we introduce a way to calculate a collusion probability-weighted reputation to reduce falsely cumulated reputation by malicious collusions. Through simulations, we show that our approach is enough to detect colluders forming a clique and shows better performance than heuristic-based approach in terms of authentic file download and reputation management. I. INTRODUCTIONA successful deployment of P2P in various networking applications has been driven by outstanding natures of P2P including anonymous and openness. However, the open and anonymous nature leads to various threats by non-honest peers [2] such as selfish behaviour (i.e., free-riding) utilizing P2P's resources without contributing appropriate amount of resources and malicious attacks trying to exploit P2P networks for their malicious purposes like worm dispatching [3].As one approach to ensure continuous success of P2P by preserving the P2P's good natures, reputation management systems have been proposed [5]-[10]. The reputation management system allows the peers to identify reliable peers by providing useful information about the peers. Thus, the success of the reputation management system is measured by how accurately the calculated reputations predicts the quality of future peer behaviour. However, the reputation management system is facing various attacks including self-promoting, whitewashing, slandering, and denial of service [4] that are need to be solved for successful work of the reputation management system.Among various attacks, colluding attack (as one type of selfpromoting) is more likely to threat the reputation management system than other attacks since colluding peers fully aware of