Reputation aggregation methods are used in unstructured peer-to-peer(P2P) networks to evaluate the trust-worthiness of participating peers and to combat malicious peer's behaviors. In reputation aggregation methods, each peer collects local scores by each transaction and calculates global scores by aggregating local scores. Using the global scores, individual peer can interact with reliable peers. GossipTrust is proposed as a reputation aggregation method for the unstructured P2P networks, and it can avoid transacting with peers with simple malicious behavior. Although, there are tactical malicious peers, which provide not only inauthentic files but also authentic files in each file request in the networks. GossipTrust has a problem that these peers are able to provide inauthentic files effectively. In this paper, we propose a reputation aggregation method called "Trust Based on Provisional Value(PTrust)." PTrust enables peers to send occasional messages to lower the global scores of malicious peers as soon as they receive inauthentic files. By using PTrust, we can effectively combat tactical malicious peers and solve the problem in GossipTrust. In computer simulations, PTrust has been shown to decrease the number of inauthentic file downloads compared with GossipTrust.