More than half of today's Internet traffic is generated by P2P applications, and most of which is the BitTorrent file sharing traffic. The random neighbor selection mechanism of BT puts unprecedented pressure on the network operators and service providers. In this paper firstly we give a theoretical analysis on the impact of neighbor selection mechanism on BitTorrent performance as well as network resource cost. And then based on network performance prediction techniques, we propose a network performance aware BitTorrent traffic optimization mechanism NPA-BT, which uses network performance information to help overlay constructing. Simulation results show that NPA-BT can select peers in the same AS network and with good performance as neighbors, and thus the BitTorrent download time and the cross AS network traffic are all decreased.