In this paper, we provide a comprehensive treatment of QoS guarantee for mobile streaming applications through a contract-ruled approach. We envision a peer-to-peer streaming system as a QoS trading market, where the involved parties, Services Provider (SP), End User (EU) and assisting peers, are all real economic entities that are organized with contractual constraints for achieving a stable and guaranteed QoS output. The QoS trading in the market is classified into two parts, a basic contract that establishes the business agreement between an interested EU and a SP and a subcontract that achieves a desired joint QoS output. The proposed scheme can benefit all parties.